Amulet of Vigor - one of the cards that makes this deck really shine, eliminating the downside of many of my lands, while also giving my titans +2/0 and haste (plus an extra land!). Multiples of this ramp me really fast, and is a force to be reckoned with.

Summoner's Pact - pretty much 4 more copies of Primeval Titan or whatever creature is needed, but be careful with the upkeep (I like to account for them potentially destroying any of my lands with a Tectonic Edge or the likes)!

Scapeshift - capable of winning games with Dryad of the Ilysian Grove or Amulet of Vigor + 6 lands out by finding Valakut, the Molten Pinnacle or setting up a (usually) game winning Cultivator Colossus by finding Bouncelands + Tolaria West. With only 5 lands and Amulet of Vigor out, you can run away with the game by transmuting into Primeval Titan in pretty much the same way, depending on which lands you have to sacrifice. It can also function as mana-fixing or ramp depending on the situation as well. Not really the greatest thing in multiples, since most of our lands are important utility lands, but definitely a lot of synergy with the deck overall. It generally ends up a cheaper Cultivator Colossus or Primeval Titan, but sometimes one of your lands in play were important, so it's not always game winning on the spot (we help to mitigate this by running both Hanweir Battlements   + Slayers' Stronghold). It's also pretty horrible without Dryad of the Ilysian Grove or Amulet of Vigor, so it isn't ideal against decks with interaction.

Bring to Light - tutor for any 3 drop or lower creature, or even Scapeshift to tutor into a threat with Tolaria West. It essentially works as Eladamri's Call where you put the card into play, but instead of finding something larger than 3 CMC, we find Scapeshift instead. Without Amulet of Vigor for the Scapeshift line, we will look for Valki, God of Lies  to play as Tibalt, Cosmic Impostor  or Dryad of the Ilysian Grove / some other creature.

Honorable Mentions

Fae of Wishes / Wish / Glittering Wish - even though Fae of Wishes is technically a creature, it's adventure mode is a non-creature spell, which is what we are interested in mainly. This is the most relevant of the wish cards, since we can still cast it for the adventure mode when hidden away, and it has extra synergy when hit with Turntimber Symbiosis   for a 4/7 flier. You could also abuse the discard mechanic if you were running some grave tech like The Gitrog Monster and/or Crucible of Worlds. Wish is just the most flexible wishing card, letting you grab literally anything. However, it forces you to use it the same turn, so often it isn't as ideal as some other wish options. Glittering Wish is a tutor for anything multicolored in the sideboard.

Tibalt, Cosmic Impostor  / Savor the Moment - Savor the Moment is one card that is mainly for enabling other cards, but it usually isn't horrible to draw into either. Skipping your untap step sounds like this card doesn't do much, but it is actually a nice fit for this deck (worst case, an overpriced Explore). It essentially reads as: "Draw a card. You get an additional combat step. Reset your lands played for the turn to 0." That's actually crazy value for the deck, especially late game. Tibalt, Cosmic Impostor  serves the same purpose, but is less reliant on having other cards to make it good. It creates card advantage, and can be spot removal, being a very versatile target. It is also conveniently a target at 2 colors, so it can always be found. This makes both a great card to hideaway with Mosswort Bridge, or to find with Bring to Light if you don't have the presence for a Scapeshift line.

Growth Spiral / Explore / Growth Spasm - The main benefit to Growth Spiral over Explore is if you have any lands that can be used in response to something else, for example Sejiri Steppe for protection of a creature, any bounceland as protection against land removal, or an activated Valakut, the Molten Pinnacle that you can use as removal for your opponent's creatures/planeswalkers. You can also use it for extra information in response to a Simic Growth Chamber trigger, allowing you to bounce a different land, or sneak in the one in your hand to reuse, etc. The extra color restriction can backfire, so it is a slight trade-off. Growth Spasm lets you essentially ramp 2 extra mana, and gives a blocker in a pinch.

Manamorphose - color-fixing where the main downside is not knowing what card it "really is" when considering your hand for a mulligan. It also doubles as a pseudo-answer for Blood Moon as a happy coincidence.

Eladamri's Call / Threats Undetected - Even though this isn't quite as good as Summoner's Pact in most situations, they are both solid tutors. Eladamri's Call lets you tutor for any creature, which is often the missing piece in the puzzle (solid turn 2 if manabase supports it). Threats Undetected will let you find two relevant creatures, which lets you go over the top of your opponents (requires enough targets).

Escape to the Wilds - Pretty close to a draw 5 card with an extra land play tacked on, since we can use the extra lands unlike a lot of other decks.

Heartless Summoning - this is very comparable to Amulet of Vigor in what it does for the deck. It allows me to play my creatures faster and for less. However, unlike Amulet of Vigor it is very bad in duplicates, so 2 copies at most. It also doesn't synergize very well with Mosswort Bridge so definitely a far worse acceleration card.

Karn, the Great Creator - This is another "wish" option, he's just in an awkward spot of the mana-curve. 4 CMC is an awkward spot for this deck, as you generally jump from 3 to 5 or more relatively quickly. However, Karn, the Great Creator mitigates that with his -2 which let's you grab any artifact from your sidedeck and potentially play it in the same turn helping to have an outlet to spend your mana on. He also can be very relevant artifact hate, both with his passive and +1 which is lowkey removal for any artifacts that are or / .

Primeval Titan - the main creature this deck revolves around, quite the force to be reckoned with. It often comes out as a 8/6 haste trample vigilance, after fetching Boros Garrison and Slayers' Stronghold with at least 1 Amulet of Vigor out (each multiple means it gets another +2/0). Also, even if it gets removed, you get the land triggers, so sometimes playing into removal is the correct play, and ignoring haste.

Cultivator Colossus - This is basically Primeval Titan's big brother when your hand is mana flooded or a utility land you need is in your hand instead of your deck. It doesn't quite go infinite with bouncelands since you need to fully resolve Cultivator Colossus's ability before resolving anything else, but it is definitely a way of making the most of mana-flooding, letting you draw into things to spend that mana on. This is significantly worse than Primeval Titan if you don't have at least 2 lands in hand. If you have 4+ lands in hand, I would say it is generally better. With 3 lands in hand, they are generally equivalent, with certain boardstates preferring one option over the other.

Arboreal Grazer / Sakura-Tribe Scout / Sakura-Tribe Elder - First there's the 1 drop creatures with slightly different uses. Arboreal Grazer is actually fetchable ramp in a sense, because it can net you 1+ extra mana in a lot of situations with a Summoner's Pact (not a suggested path against interaction). It also has reach and makes a decent blocker afterwards. Sakura-Tribe Scout is a little different, since it does nothing the turn you cast it. However, the tap effect is not only reusable every turn, but it can also let you play lands as a response to actions, similar to Growth Spiral (for example Sejiri Steppe for protection of a creature, any bounceland as protection against land removal, or with an activated Valakut, the Molten Pinnacle that you can use as removal for your opponent's creatures/planeswalkers). Sakura-Tribe Scout is also capable of ramping you to a turn 4 Primeval Titan on its own, with no other acceleration in hand. Sakura-Tribe Elder is something you can block with, then sacrifice it, for "ramp" the turn you draw it, with the basic land being resilient to removal/hate and helps a bit against Blood Moon.

Elvish Reclaimer - Surprisingly, this versatile guy is really good at just about everything. It can act as a psuedo-extra land card, a land tutor, a sizable threat to stabilize when you have nothing. This is a 1 drop that scales up as the game goes on, and can also be used as a tutor for the land you really need (really good at finding your missing bounceland or Urza's Saga). You can also use the activated ability in response to the triggered ability of Urza's Saga's 3rd chapter to basically replace/upgrade your land in addition to searching for your artifact of choice.

Azusa, Lost but Seeking / Dryad of the Ilysian Grove / Wayward Swordtooth- Azusa, Lost but Seeking is two additional land drops per turn, which enables Primeval Titan the turn after pacting and having 0 open mana with an amulet and bounceland. However, she is legendary, so you never want more than 1, which is why I generally run 1-2 copies of her. Dryad of the Ilysian Grove is an additional land drop every turn, mana-fixing, and a solid 2/4 body for 3 mana. It also combos with Valakut, the Molten Pinnacle as a win-condition. Wayward Swordtooth is resilient to red removal due to its high toughness. Its high power can also help with Mosswort Bridge even if it can't attack or block. Definitely the weakest of the three in most situations, but it does have its scenarios.

You can't really fill the deck with too many utility creatures and Elvish Reclaimer is a borderline card that is sort of a utility creature / ramper in certain scenarios, so if you are running it, make sure you keep utility creatures to an absolute minimum.

Honorable Mentions

Eldrazi Displacer - This card is capable of a lot more than you think at first glance. Its activated ability is where we are interested, which is flexible, because it can be used on both our creatures, and our opponents creatures. Also, since we have Amulet of Vigor this can also act as a psuedo-vigilance for us, while tapping down our opponent's creatures. It is usually best served when activated on one of our creatures with an ETB trigger, but there's other combat trick / protection usages as well. Of note, you can blink your titan, finding to loop for deck thinning, basically guaranteeing a hit with Mosswort Bridge later (start with Crumbling Vestige + Valakut, the Molten Pinnacle if you have Dryad of the Ilysian Grove to sneak in some extra damage). You can also use it similarly to refill your hand, or even find protection like Sejiri Steppe or some other utility land at instant speed. This can enable a lot, while also essentially being a double blocker on its own, so it isn't strictly a combo piece. It is also capable of ramping with multiple Amulet of Vigor or drawing loads of cards with Cultivator Colossus.

Omnath, Locus of Creation / Soul of Windgrace - if you can somehow support the greedy casting cost in here, both are worthy of inclusion. Omnath, Locus of Creation is one of the best landfall creatures ever created, and Soul of Windgrace can be seen as a creature version of a planeswalker, for all intents and purposes. I tried them for a while, but found if I could get to their manacosts, I could get to almost as easily, so I found there's generally better options by the time we have access to the casting cost.

Tireless Tracker / Courser of Kruphix / Radha, Heart of Keld / Augur of Autumn / Ramunap Excavator - Tireless Tracker is slightly different than the rest, but I would say it probably fights for this slot the most. It is a form of card advantage that is capable of growing itself, although at the cost of investing further mana, unlike some of the following options. Courser of Kruphix is good at letting me eliminate land draws, which can really be a problem sometimes. It also is Lightning Bolt proof, which can be huge sometimes. Lifegain from lands hitting play is just icing on the cake. Augur of Autumn / Radha, Heart of Keld does basically the same thing as Courser of Kruphix, but they are geared more towards grindy control matches instead of aggro decks, since they don't broadcast info. Augur of Autumn can let you play creatures from the top of your deck as well if you have 3 creatures with different power, while Radha, Heart of Keld can pump itself up to be a sizable threat. Ramunap Excavator is slightly different from the others, as it enables playing lands from our graveyard, instead of the top of our decks. Something that is also very relevant for them all is that its 2+ power can be just what's needed for that Mosswort Bridge activation.

Springbloom Druid / Mina and Denn, Wildborn / Oracle of Mul Daya / The Gitrog Monster - Springbloom Druid may seem slightly different than the rest, but for all intents and purposes it accomplishes basically the same thing as an extra land play for the turn, while helping against Blood Moon. Mina and Denn, Wildborn is a fatty that can give trample, while Oracle of Mul Daya is built in card advantage and an alternative way to use our land drops. The Gitrog Monster is an additional land drop, card advantage, and a decent body all wrapped up into one package. It's also nice to have a relevant 5 CMC card to fetch with a Summoner's Pact when I fall just short of the 6 CMC Primeval Titan - the only downside is sometimes it forces me to pitch a land I'd rather not or it ends up keeping you in a 1 mana short cycle where you are stuck at 5 mana due to having to sacrifice a land every turn.

Lotus Cobra / Tireless Provisioner - Lotus Cobra helps to produce extra mana of any color with all of our land drops. Lotus Cobra is more of a combo deck option, and can almost be viewed as a worse Amulet of Vigor for mana purposes. Another thing that is relevant is that its 2 power can be just what's needed for that Mosswort Bridge activation. It can also be found with a Summoner's Pact when you opponent is completely tapped out, and you could cast a Primeval Titan already, since you'll get back your two mana on the first fetch. Tireless Provisioner is very similar, except it also can create food tokens, and "store" mana across turns, since you don't have to use your Treasure tokens right away.

Questing Beast - surprisingly this card is another solid option for a lower CMC bomb. It makes a good blocker even as you chip away at their life points due to vigilance. It can also chip away at their swarm, or deter them from swinging with the fatty due to deathtouch.

Nethroi, Apex of Death - a solid body on a 5 CMC with deathtouch and lifelink, basically a mini Wurmcoil Engine. However, Nethroi, Apex of Death really shines for its mutate ability against removal. If you have enough mana, instead of finding another Primeval Titan with your Summoner's Pact that you transmuted from Tolaria West, you can find this guy and bring back most if not all of what they removed.

Woodland Bellower - pretty decent option with the variety of 3 CMC and below creatures that are viable in this deck.

Tolaria West / Turntimber, Serpentine Wood  / Port of Karfell - Tolaria West can transmute into another Summoner's Pact for whichever creature fits your situation best. It can also become any land, a Chalice of the Void or Engineered Explosives if either is in the deck (or any other 0 CMC sideboard card). Turntimber, Serpentine Wood  is basically a land that doubles as a threat with any bounceland later in the game. It is very comparable to Tolaria West in functionality, except it cannot be fetched by Primeval Titan and is a much preferred untapped green source when needed (notable when trying to stretch your manabase a little too much). Port of Karfell is basically recursion of a titan that's uncounterable in most situations. It is a decent option for the slot in more control oriented metas if you are in those colors.

Valakut, the Molten Pinnacle - this is actually a win-con with enough land drops and Dryad of the Ilysian Grove. Due to the Bouncelands being able to bounce themselves, you are generally able to utilize all your extra land drops on each turn, so this damage can add up really quickly. It can be used to clear the board or to do direct damage.

Crumbling Vestige / Vesuva - these are basically a mandatory include because of all the great plays it enables. You can produce mana without an Amulet of Vigor out off a Primeval Titan fetch. You can also go +1 land with Crumbling Vestige + Slayers' Stronghold instead of Boros Garrison + Slayers' Stronghold when you have Dryad of the Ilysian Grove out, and sometimes that one land can be the difference that enables more burn damage with Valakut, the Molten Pinnacle. It can also be used to leave an extra mana open after attacking, by getting Hanweir Battlements  allowing you to have at least 5 open mana after attacking when finding 2 bounces (perfect for Bring to Light in your second main once you have more lands). Vesuva is probably the best land in the deck, as it can be pretty much anything. In addition to copying anything (opponent's lands included), you can play it untapped copying nothing with Dryad of the Ilysian Grove out to produce mana, an often overlooked line of play.

Boseiju, Who Endures - One of the best utility lands for the deck, since we can easily bounce it back to hand, making it fetchable removal for artifacts, enchantments, and even troublesome lands (essentially a broader version of Ghost Quarter if you don't plan to use it on yourself). It does technically ramp your opponent in most cases, but that's a fair tradeoff for eliminating a more troublesome permanent.

Cavern of Souls - good for control matches, and it can also be useful for colored mana when trying to hard cast more color intensive creatures like Sigarda, Host of Herons. Since we have a lot of bouncelands in here, we can change the creature type named almost as often as wanted, so this is almost like an Ancient Ziggurat that can tap for colorless when not casting creature spells.

Slayers' Stronghold / Hanweir Battlements  - some of the best utility lands for this deck. It allows your titans to come out with haste, you can't really ask for more. Slayers' Stronghold is the better of the two, since it gives haste/vigilance and +2/0, which can be important with enabling Mosswort Bridge triggers. Having both as an option gives you a lot of flexibility in your plays, and makes us less susceptible to things like Surgical Extraction or drawing into our only haste enabling land before playing the titan. Be careful not to overcommit to non-green lands though, as you need green lands for the deck to function early. I generally suggest choosing at most 2 between these and Sunhome, Fortress of the Legion as you don't want colorless sources as much.

Sunhome, Fortress of the Legion / Sejiri Steppe / Mosswort Bridge - Sunhome, Fortress of the Legion is great to block something with deathtouch (take that Wurmcoil Engine - I live and you don't even gain life), or just double your damage. However, Sejiri Steppe accomplishes one of the main goals of this utility slot, which is to help your Primeval Titan live after attacking (against most decks), but it requires no investment of to activate. This slot is unneeded in some matches, so it can easily be sided out when it's not that relevant. Sejiri Steppe + Mosswort Bridge cover the defensive and offensive purposes of Sunhome, Fortress of the Legion in almost every situation, without the need of as much mana investment, even allowing more aggresive plays sometimes, all while being colored mana sources instead of colorless. Mosswort Bridge is the most powerful digging tool and virtual advantage this deck has to offer. Cast off anything for 1 green? Yes please! Need to find some removal because they are just stalling with something stupid and annoying? Keep copying/bouncing this until you find what you are looking for. Another thing worth mentioning is Mosswort Bridge + Vesuva in conjunction with any bounceland becomes a powerful digging tool (hide multiple land drop cards/ways to play titans to get bounces until you find your answer). Also, this lets us cast Fae of Wishes or any other Wish variant at instant speed if it is hidden away. I used this trick a lot with Batwing Brume and Glittering Wish when Splinter Twin was prevalent to just win games during their attack phase when they thought they'd won. If you are running Mosswort Bridge, it is basically required to run Slayers' Stronghold + Boros Garrison alongside it, allowing you to get to 10+ power far more consistently. Because of those mandatory inclusions, Sunhome, Fortress of the Legion is a basically equivalent option as well, being in the same colors. Also worth mentioning is that you can use the hideaway of Mosswort Bridge to exile cards you don't want to draw, if you whiff, or don't plan on activating it prior to bouncing it back to your hand. After a shuffling effect, this deck-thinning helps increase your odds of hitting cards you want by a few percentage points.

Castle Garenbrig - this is decent as potential ramp if you run enough fetches + Forest Subtypes that it can consistently come out untapped. Not really worth forcing into the deck, but it can be viewed as +2 for mana calculations in a lot of scenarios, so definitely a solid include.

Urza's Saga - I've finally given in and invested in a playset of Urza's Saga due to the early plays it enables game 1. However, I don't think it is good enough of a card that you necessarily need to run a full playset (I fluctuate between 3-4 copies). If you have to rely on Urza's Saga to find Amulet of Vigor, your hand wasn't amazing anyway. I still maintain it is over-hyped and super vulnerable to removal (you basically concede against an early Blood Moon, Magus of the Moon, or Force of Vigor). However, one thing the saga does well is allow for a double amulet more often (making it a solid turn 1/2 land). One additional upside the Saga does have going for it is the negatives aren't really preyed upon until games 2/3 of the match making it a powerful game 1 tool. I also don't run Expedition Map alongside this, as it's only decent alongside a full playset of sagas (I don't think it is worth it if not playing 4 copies), and you'll often find Amulet of Vigor instead in almost every situation anyway. I think Elvish Reclaimer does a much better job than the map, all things considered (way more versatile + puts the land into play instead of hand, so it enables interactive plays).

Forest / Snow-Covered Forest - basic lands are needed, enough said. It stops Path to Exile from being as devastating, and allows some powerful interactions by targeting yourself with Ghost Quarter, along with giving hope against Blood Moon. I typically like to use white bordered basics to make fetching easier (you can fan out your deck instead of going card by card) if going with fetchlands, just a little trick I like to use to help speed up games.

Selesnya Sanctuary / Golgari Rot Farm / Simic Growth Chamber / Gruul Turf - green bouncelands, the main thing this deck revolves around. It is a good idea to keep one of these in hand at all times (or a Vesuva to copy one) to make the most of your multiple land drop potential. The only time I play my last one in hand is if it means I can hard cast a bomb that I feel will get through when I untap next turn. Otherwise it is best to keep one in hand with the potential of drawing extra land cards, allowing you to "go off" in a lot of cases (considering you have either an amulet in play, or dryad/azusa in hand, and are just waiting on the other piece).

Honorable Mentions

Verdant Catacombs / Overgrown Tomb / Zagoth Triome - Fetches / Shocks / Triomes just enable better mana fixing, landfall triggers, and deckthinning. The triomes can also become card draw when bounced if needed. You should generally pick 5 color lands or fetches, it's a little hard to support both. I generally go with fetches alongside at least one shock, because you still have all the untapped potential, but can choose for them to be tapped sources as well, to make the most of multiple Amulet of Vigor lines.

Aether Hub / Gemstone Mine / Tendo Ice Bridge / City of Brass / Mana Confluence / Vivid Grove / Nurturing Peatland / Yavimaya, Cradle of Growth / Reflecting Pool / Twilight Mire - Gemstone Mine is probably the best of the rest, since you can use it twice then bounce it to reset its counters. Aether Hub is a strictly better version of Tendo Ice Bridge and is next in line, since you can bounce it and replay it without using the to stack multiples for later. City of Brass deals 1 damage whenever tapped and Mana Confluence allows you to pay 1 life for any colored mana, which are very important distinctions, and here's why: City of Brass will deal damage to you no matter if you have a replacement effect on it or not, so things like Yavimaya, Cradle of Growth or Dryad of the Ilysian Grove don't help you. However, since it is damage, and not loss of life, it doesn't conflict with Elderscale Wurm or Worship. Mana Confluence is the complete opposite, so if you use it's mana ability with Elderscale Wurm out and go under 7 life, his ability doesn't prevent it, and you are basically vulnerable again. Vivid Grove comes into play tapped, essentially making it the worst of the bunch as it can sometimes takes away strong turn 1 potential. The draw-land cycle of cards like Nurturing Peatland is one land that actually isn't terrible to draw when mana-flooded. Yavimaya, Cradle of Growth gives all lands the ability to tap for which allows the deck to stretch it's manabase a little thinner. The filter cycle like Twilight Mire helps manafix if you have a lot of non-green double cost cards. Reflecting Pool is basically a psuedo-5-color land that makes cards with multiple of the same color cost a lot easier to cast. It gets exponentially better with more multicolored lands, and is quite nice in greedy manabases like this one.

Lair of the Hydra / Oran-Rief, the Vastwood / Turntimber Grove - Lair of the Hydra is a green land that is untapped when you need it early, so basically another Forest. However, this is a manland, which can let you pump your total power on a titan fetch, or become a threat on its own. Oran-Rief, the Vastwood is great with the prevalence of Unholy Heat in the meta, being able to protect a Primeval Titan by just leaving it untapped, or just pumping creatures, sometimes enough for a Mosswort Bridge activation. Similarly, Turntimber Grove works pretty much the same, but it is better on the titan fetch (being able to pump and tap for ), but is not reusable every turn without other support.

Lotus Field / Flagstones of Trokair - Typically these two lands are a package deal, as they are a nice fetch-combo when you need lots of mana. These are capable of producing 5 mana per Amulet of Vigor and still netting you +2 lands for your Valakut, the Molten Pinnacle triggers when fetched. Lotus Field is also capable of pumping our Elvish Reclaimer on its own (can be used after blocking). Flagstones of Trokair being a non-green source, and Lotus Field decimating your land count on their own are substantial downsides to consider, so you need enough synergy to justify inclusion of these. You'll often see these alongside Elvish Reclaimer due to the great synergy.

Crystal Grotto / Temple of Plenty - scry lands. If there's not a whole lot of early plays that need a turn 1 untapped land in your 75, the scry outweighs the slower land in most cases. Crystal Grotto is Zhalfirin Void 2.0, which can let you mana-fix as well, now giving you a relevant untapped version of the temples.

Ghost Quarter / Field of Ruin - land destruction that can double as another copy of a Forest if facing hate like Blood Moon. It's worth mentioning - don't hesitate to blow up your own lands with Ghost Quarter for landfall or The Gitrog Monster triggers - this has saved me on numerous occasions. It is actually a 3 mana land when fetched by Primeval Titan with Lotus Cobra or Tireless Provisioner out (two landfalls + green). It also allows you to draw 2 cards with The Gitrog Monster (one on the sacrifice cost, then one on the land destruction). In most cases, Boseiju, Who Endures is superior due to the added flexibility, but these are still solid options worth mentioning, especially if you have a way to recur lands from your graveyard. Ghost Quarter is especially good against something that lets you play lands from the graveyard, completely decimating your opponent's access to splash colors.

Khalni Garden / Radiant Fountain / Kabira Crossroads / Blossoming Sands cycle - chump blocker or lifegain slot. Sometimes that is enough to turn the tide of battle, so it isn't to be under-estimated as a utility option. I generally go with Khalni Garden / Kabira Crossroads if I have enough space (can potentially filter the with Wooded Bastion).

Blast Zone - This is more of a utility land to deal with the decks that pump out tons of low cost creatures/board presence. This is basically an Engineered Explosives attached to a land. It's biggest drawback is that it can't hit 0 cost permanents, since it enters with 1 counter on it already. You can also use it to blow up some big hard to remove threat with hexproof/shroud if you have enough mana.

Arena / Quicksand / Eiganjo, Seat of the Empire - targeted removal with their own respective pros and cons. Notably, Arena + Sejiri Steppe have decent synergy as a Primeval Titan fetch package. However, Arena not producing its own mana is the main thing holding it back, so you should consider it as more of a fetchable creature removal than a land if trying to fit it in. Quicksand is probably my favorite of the bunch.

It's important to be realistic about the deck, and point out what are the weakest/most unneeded cards currently. I'll point out what I want them to do for me, and why I don't think they are "ideal." Feel free to discuss other options or suggest replacements

Valki, God of Lies   - He's mainly in here as a Bring to Light target when Scapeshift isn't good. It just feels bad to draw into this particular card. I need something for this slot, and the fact that he can be found on only 2 colors is quite nice. He is definitely overpriced at 7 mana, realistically powered around most 5 mana planeswalkers, but a slightly worse option than other big mana avenues. This could be replaced with some 4 CMC powerhouse like Omnath, Locus of Creation / Soul of Windgrace / Phyrexian Obliterator or even something that is conditionally good (at different times from Scapeshift) like Savor the Moment. Nothing I've tried has felt correct, so I'd definitely say it is the weakest slot right now.

Wayward Swordtooth - Honestly, most of the time this is just a play an extra land card that can't block or anything. It is relevant to enable Mosswort Bridge sometimes, but most of the time you would much prefer something with added value like Dryad of the Ilysian Grove or Azusa, Lost but Seeking. I do prefer the reusability every turn over something like Arboreal Grazer for the slot, and it's also worth mentioning I don't want to run a second Azusa, Lost but Seeking because that legend rule comes up so much more often than you'd think it would due to our digging power of Mosswort Bridge.

Our second copy of Tolaria West - One copy is generally enough to win, unless playing against a more interactive deck. Drawing into it generally feels bad, and this is probably one of the first lands I'd swap for something else. For example, Port of Karfell fills the same basic slot in a better way for most situations, the only thing it doesn't do better is if I transmute for a second titan when Tolaria West got me the first one, or want to Scapeshift away the first Tolaria West and still be able to transmute a second one. Also if their removal is exiling, it is a definite feels bad moment, I would probably make the swap if Solitude wasn't a thing.

Mosswort Bridge is also a land that isn't completely necessary. I run it as my "I win" land over Sunhome, Fortress of the Legion mainly since it is less mana investment and opens up more interesting lines of play. However, it is reliant on more than just mana, so depending on how interactive the meta is, you may want to run Sunhome, Fortress of the Legion or neither option (the slot isn't 100% needed honestly, it is just nice to have).

Sideboard
Carnage Tyrant - sometimes this card feels like overkill against control. It may not be completely needed, but with skimping on some of my other anti-control options, uncounterable is a crutch I like to lean on.

Culling Ritual - probably one of the most irrelevant sideboard options, since there's not a whole lot of swarmy decks anymore, and it hits our own Amulet of Vigor. That being said, when it is good, it is amazing. It does cover us against most artifact/enchantment decks as well, since they generally center around lower CMC options. Additionally, it allows us to play a bomb with the ramped mana after clearing the board and is even findable with Bring to Light which gives it some extra value.

To make things a little easier, I marked all the cards I sometimes side out in various matches with the tag #flexslot if you look at custom categories.

First of all, when heading into game two, I usually decide what to side out first. I evaluate which cards are less than optimal. Arboreal Grazer gets cut to 1 copy against decks that don't plan to attack much. Things along those lines, where you take out suboptimal cards. Something like Fae of Wishes will almost always go out after the first game, since we are moving most of its relevant targets in (sometimes a flying blocker can be useful).

After all that, I decide which sideboard cards could prove useful in the match, and move them in. Afterwards, I adjust to make my sideboard as close to 15 cards as I can, whether I need to remove more "filler" slots like Summoner's Pact / Eladamri's Call, or if I have to move in some of the "better" cards I tried to side out (dependent on the match).

Another option is removing some utility lands that don't really serve a purpose in the match (Cavern of Souls and Sejiri Steppe) depending on the deck you are facing. I generally don't suggest going down below 28 lands with the deck. Also worth mentioning is don't stress about getting exactly 60/15; there's times I will play with a 61 card deck in game 2 (usually against some weird homebrew where I want more options and lots of our sideboard seems relevant).

Lastly, you can always include some sneaky tech like Hokori, Dust Drinker / Worship / Elderscale Wurm if you have the space (I ran Elderscale Wurm mainboard for a while before modern became a removal heavy format). If you're facing anything with a more aggro build, Hokori, Dust Drinker isn't the best option. For example, puresteel decks can be seen as a combo deck, but they are a more aggro combo deck, where they get early board presence. Hokori, Dust Drinker is not a good option against decks like these, since they can still keep applying pressure even without mana with a quick start, and since a large majority of the meta falls into this aggro-esque category, Hokori, Dust Drinker isn't really poised as a solid sideboard option currently. However, the surprise factor of an obscure card like this definitely pays off sometimes, so that's just something to keep in mind. I am talking about obscure cards in general being a solid enough option if you have the extra slots, and am just Hokori, Dust Drinker as an example. If you feel they will do enough in your meta to justify inclusion, feel free to experiment a little.

Tutor for any 0 CMC
Tolaria West + Simic Growth Chamber

Removal
Boseiju, Who Endures + Bounceland

Creature Removal
Arena + Lotus Field
Arena + Sejiri Steppe

Reusable Protection/Unblockable with Sakura-Tribe Scout
Sejiri Steppe + Bounceland

Extra Burn damage with Dryad of the Ilysian Grove and at least 4 lands out
Valakut, the Molten Pinnacle + Hanweir Battlements  for haste (Vesuva can't copy a land entering at the same time, so fetch it after attacking along with the 2nd Valakut, the Molten Pinnacle for 3 copies). You can also target yourself with Ghost Quarter for an extra landfall trigger.

+2/0 and Haste/Vigilance (another fetch after attack)
Slayers' Stronghold + Boros Garrison

Free Plays with 10 power (if you have extra landdrops left, it gets really broken)
Mosswort Bridge + Bounceland - you can stack triggers so you can resolve and activate the hideaway before bouncing Mosswort Bridge back to your hand
Mosswort Bridge + Vesuva allows you to double down on the hideaway almost always (you can't copy the Mosswort Bridge but you will almost always find at least an extra land creature on the first hideaway). This means you can copy a bounce with Vesuva initially, to be able to replay it (now copying Mosswort Bridge) for a second chance at finding a finisher.

Haste without Amulet
Hanweir Battlements  + Crumbling Vestige or a Bounceland and extra land plays to replay the Hanweir Battlements  and give the titan haste

Simic Growth Chamber + Tolaria West - Tolaria West + Simic Growth Chamber lets you transmute into a 0 CMC card
Boseiju, Who Endures + Selesnya Sanctuary - Boseiju, Who Endures + any bounceland lets you have removal for any artifact/enchantment/land
Dryad of the Ilysian Grove + Valakut, the Molten Pinnacle - Valakut, the Molten Pinnacle + Dryad of the Ilysian Grove allows everything to be mountains, so Valakut, the Molten Pinnacle + Hanweir Battlements  becomes one of the better fetch options