This deck is designed to put out a lot of Merfolk with +1/+1 counters with tempo in mind, and then follow up with a variety of finishers to break stalls.
Summary of Deck:
Creatures(27):
4x Deeproot Elite- A better Metallic Mimic for Merfolk because you choose where the counter goes, good to get counters on creatures and setup for later.
3x Herald of Secret Streams- This is one of the finishers. Wait to play him until you are ready to attack with a bunch of buffed Merfolk, he will be a big target when he comes out so keep Blossoming Defense around.
4x Jadelight Ranger- A good Merfolk that buffs itself and most importantly filters for your finishers.
3x Kumena, Tyrant of Orazca- Her varied abilities enable you to do a lot to break stalls and finish the game either right away or over time. Use her abilities at their end step so you can block or deter your opponent from attacking.
4x Merfolk Branchwalker- Mini Jadelight Ranger, good for 2 mana.
4x Merfolk Mistbinder- Buffs all your merfolk, making your board state scarier while also being a good target for your opponents removal.
4xMetallic Mimic-Essentially same purpose as Deeproot Elite, get those counters on creatures.
1x Tishana, Voice of Thunder- Tishana is another card for breaking the stall and filling your hands with goodies.
Spells(11):
4x Blossoming Defense- Used to protect your wincons or on the rare occasion deal extra damage.
2x Deeproot Waters- Gives you more Merfolk and most of all fodder for Kumena's tap ability. Useful in some but not all scenarios.
1x River's Rebuke- A good finisher when other plans aren't working out. Only 1 because it's a dead card in many situations.
4x Unsummon- Good for stalling out the game, getting rid of threats temporarily, or if you're unblockable plan isn't going well.
Lands(22):
3x Botanical Sanctum
8x Forest
7x Island
4x Unclaimed Territory
Sideboard(15):
(Note, this is subject to change as the meta becomes visible, or depending on your local tournaments.)
3x Kopala, Warden of Waves- Good to further deter your opponents from destroying your wincons.
2x Nissa, Steward of Elements- Her +2 is great for filtering for your win cons, her -0 is good for playing them, and her -6 is good for dealing 10 damage in the air. I wouldn't mind putting her in mainboard.
1x River's Rebuke- Double your chance of getting your alt win con.
4x Spell Pierce- Protects your creatures or deals with your opponents annoying spells(for example, Torment of Hailfire which might become big.)
2x Tishana, Voice of Thunder- My least favorite pick for sideboard. Is only good in scenarios where you need to guarantee your win con but mana cost is too high to be reliable. I'm willing to take suggestions for sideboard.]
Maybeboard Discussion:
Silvergill's Adept is a really good card but I don't think it exactly fits in with the deck where I can filter and get +1/+1 counters with the explore mechanic.
Shaper's Sanctuary is very good but I have found it's better to protect your merfolk and build your deck around having enough card draw.
Conclusion: This is the third draft of this deck that I have been testing on Untap, I took some tips from Lee Shi Tian in this article(http://www.mtgmintcard.com/articles/writers/lee-shi-tian/merfolk-in-rix-constructed). I think this deck will be powerful enough in standard now that we have all the tools. There is definitely room for improvement and I implore you to experiment with it and tell me what you think I did wrong or right.
Update(1/7/18):
After some playtesting I found some trouble with some enchantments and artifacts, specifically Ixalan's Binding and Azor's Gateway(which has a strange little control deck built around it). The only cards that can be used to counter these are Slice in Twain and Appetite for the Unnatural. Both are alright but not as ideal as Fragmentize and I'm not sure if it is worth it.
Update(1/8/18):
The Sideboard was changed to include Naturalize as a way to counter any possible decks based around a specific enchantment or artifact. Considering running Heroic Intervention over Blossoming Defense, or putting it in the sideboard. Also to remind myself, Spell Pierce is very necessary in sideboard to protect against board wipes(like Hour of Devestation) or big spells that threaten the merfolk.
