Here I'll go over the Maybeboard for this list to explain why these cards didn't make the cut for me and why they might for you.
River Kelpie:This card is pretty similar to Grim Haruspex and Midnight Reaper, but with persist (meaning it can trigger off of itself) making it much more difficult to put down - however, if it is killed for good before the end step then we won't get to draw. Because +1/+1 counters cancel -1/-1 counters, this goes infinite with a sac outlet and Olivia, Mobilized for War or Metallic Mimic to draw through your whole deck, although that's not the intended purpose of either card. Ultimately I cut River Kelpie for two reasons: it's CMC of five is often too slow, and it only really draws cards while the engine is online. I may reevaluate this choice in the future.
Dack's Duplicate: The card has promise, but I'd ultimately I'd rather recur an existing card via Marchesa than copy it's ETB trigger via Dack's Duplicate.
Pyreheart Wolf: Much harder to kill than Herald of Secret Streams but also gives worse evasion. It is 1 CMC cheaper and doesn't need our whole board to have counters, so I do intend to test it further at some point.
Rakdos Charm: Easily the best charm that we have access to, and quite possibly the best overall. Blows out token decks and certain combos, deletes a graveyard at instant speed or destroys an artifact - the former two abilities can be very punishing while the latter will always be of use. This is one of the cards I have my eye on to put into the deck in the future.
Triskelion: A powerful tech against walkers, and goes infinite with Mikaeus, the Unhallowed. However, outside of those specific situations I don't feel a Shock makes the cut over our other options, especially at 6 CMC.
Captivating Crew: A repeatable steal effect, which we can make permanent with Marchesa. Comparing this to Zealous Conscripts, we need to spend mana to steal things but we can get them before the combat step to trigger dethrone. Definitely worthy of consideration.
Hostage Taker: The fact that we need to cast the card we steal limits this cards effectiveness in that regard, but it can also be used to flicker our other cards. Ultimately I chose not to run it for similar reasons to Dack's Duplicate above - I'd rather have another trigger than try to copy one already in play.
Butcher of Malakir: A Grave Pact on an evasive creature. Ultimately the high CMC made me cut this card, I don't feel the extra pressure on life totals is worth the vulnerability to removal and increase in casting cost.
Jokulhaups: This is a stand in for any kind of MLD. These are powerful effects in Marchesa, our board can survive them while our enemies can't. Ultimately the issue I have with these sort of cards is I often struggle to kill more than one player through combat before people start setting back up, and it generates a lot of bad will. Much more viable in a more aggressive build but not great here.
Siren Stormtamer: I like the low CMC and the evasive body for easy dethrone triggers. I don't like that it only gets spells with targets. The effects that most concern me are on global enchantments, so Glen Elendra Archmage is a far better pick here.
Lim-Dul's Vault: Great card, takes too long to resolve.
Lightning Coils: I have not tested this card, but it looks to be a potent win condition. Even with just two creatures looping (a grafter and something else, for example) that's 24 power on board each turn. I definitely intend to try this.
Rune-Scarred Demon: More expensive to cast than our other tutor options while being just as threatening. The effect is strong, but I don't think it is necessary at that high a CMC.
Corpse Augur: I don't feel the deck needs more draw spells, but if I did this would be one of my first considerations. Ultimately the fact that Corpse Augur has a death trigger rather than an ETB trigger is why I chose other cards over it.
Smothering Abomination: Costs more than Midnight Reaper or Grim Haruspex while only offering the addition of flying and a few extra stats. Combined with the forced sacrifice clause that extra makes this card inferior to other draw options.
Drana, Liberator of Malakir: Great in more aggressive builds, but for this deck the fact that she can only hand out counters on my turn just isn't good enough.
Chrome Mox: As the cheapest of the Moxen this is something I am considering to help with the early game fixing.
Ashnod's Altar: I'm happy with the number of sac outlets in the deck, but if I wasn't this would be one of the next ones in.
Herald of Secret Streams: An interesting way of pushing damage through to our opponents. Ultimately got cut from the list because it does nothing on its own and doesn't contribute to my engine in any way.
Shifting Shadow: This card got cut because it really suffers from the issues that define aura spells, and the payoff isn't that great. It does double as an amusing removal spell or a political tool.
Kiki-Jiki, Mirror Breaker: The combo goblin in the flesh, Kiki lets us copy ETB triggers. He's pretty similar to something like Dack's Duplicate in this regard, but he can also respond at instant speed, a big upside on the usual need to wait until the end step. Another card to keep an eye on for future use.
Dark Prophecy: Essentially another copy of Grim Haruspex, but as an enchantment. Ultimately the only reason I don't run this card is because of the casting cost. While that's worth it on something like Necropotence, Dark Prophecy is on nowhere near the same power level.
Burglar Rat This is one of the cards I definitely need to test. Making each opponent discard several cards per turn cycle is nothing to sniff at. Burglar Rat may well be better at this than Oona's Blackguard due to hitting everyone and not needing a board to get immediate value.
Venser, Shaper Savant An incredibly versatile removal tool, able to hit any permanent (including lands) but at the cost of only bouncing them. He's also a wizard, and can be used to protect our own permanents in a pinch. Synergy with Vendilion Clique to ensure your target is gone for good.
Hex Parasite: A nice anti Walker tech card, and a life outlet to help activate dethrone. Very mana intensive but equally effective in the right meta. I don't feel the inclusion is necessary for the opposition I face, but could be worthwhile if superfriends are a common sight where you play.