Purging Watchkeeper does a passable Scavenging Ooze impression, and if there's an Abzan "The Rock"-style deck I'd expect to see it in the main or side. Disassociate is just a Journey to Nowhere almost-reprint with the Banishing Light functionality; I expect it to see more play than Journey does in Modern, given our lack of a Path equivalent. Serene Prayer is a huge boon to TurboGlory decks, creating a buffer of ten life that the opponent has to burn through before they can touch a glory counter. Dregs of Order is a passable control piece, shredding tokens and buying a few turns against real creatures. Martyr's Sacrifice might see play in TurboGlory or Gloryless Turbofog, but three's a lot to pay for a fog with upside that probably won't matter that much. Kasienka's playable in a white weenie/D&T deck, or in MSEDH. Overall, white's adding a few very strong cards, but not many total playables.
Resource Management, Fretport Gravetender, and Compelling Pariah all seem like interesting engines, but none of the three seem efficient enough to see competitive play. Unassuming Waif would be an excellent card in the GU Warriors deck that unfortunately will probably never exist. Exquisite Heist is a "fair" Ancestral Visions, and I fully expect it to see play in essentially every blue deck. Precarious Ledgers is potentially a key piece in a hypothetical Lantern Control-style deck, but glancing through the rest of MSEM, the other pieces all seem to be missing. Still, it's one I'm going to come back to every few months when new sets are added. Thespian's Finale seems potentially strong in a white-blue flicker deck, and might be the piece that pushes me to actually build it. Merchant's Guile is literally just Brainstorm for one mana more; it's still probably playable given how many fetchlands we've got floating around. Scatter Riches resembles a four-mana Render Silent, with the interesting upside of guaranteeing a tapped out opponent in a control mirror; I don't think it's playable, but it might see some sideboards. The Flood of Information nerf might make Empty the Streets see play alongside Vedalken Reshaper; pre-nerf, I'd have said Flood of Information was a better pick. Flee the City's a good reason to splash blue in the Poisonous deck, but I don't think the Poisonous deck's good enough even with the splash. Selective Prophecies is yet another big sorcery-speed draw spell; I'd run Merkurio's Insight over it essentially always, which is unfortunate since it's a cool and skill-testing design. Really, Exquisite Heist is the card to watch here.
Vassal of Virulence is an interesting anti-aggro option, particularly in combination with topdeck manipulation effects, but probably too unreliable to see play. False-Cure Smuggler has potential as an engine piece, in combination with the Vanishing enchantments of PFP, the vast assortment of planeswalkers in MSEM, or the poison cards of IMP/PSA. Frenzied Watchkeeper is a threat to keep an eye on, potentially seeing play alongside Lady Genevieve in a BR cripple deck. Conspiracy of Ravens is a powerful discard spell; I think it's comparable to but slightly weaker than Council's Will, and will see play. Flesh to Rot is a bit stronger than Shadow Sky, but given that Shadow Sky hasn't seen much play I don't expect it to either unless a mono-black control deck develops. Left Bereft, similarly, is unlikely to see play outside of a mono-black or blue-black control deck, but powerful within one, giving black a rare answer to enchantment-focused board states. Harriet's a decent answer to planeswalkers, and has sufficiently above-the-curve stats to see play in aggressive black decks, as well as having serious synergy potential with Frenzied Watchkeeper.
Speaking of planeswalkers, Kalemn Weller is vaguely reminiscent of Liliana of the Veil. While the +1 doesn't work with an empty hand, and she can't remove a creature the turn she comes down and survive, I think she's still a powerhouse in midrange and controlling black decks. I fully intend to try switching my main control deck back from Jeskai to Esper and including her alongside Seto San.
Balanced Blade is a second copy of White Petal Ronin for any deck that's using it for Equipment/Aura synergy instead of toughness-matters, and indeed arguably an upgrade on that front; it might see play in a RG Infect list as an excellent carrier of Kalak triggers, but its anti-synergy with Red Silk Dancer is unfortunate. Rising Riots might find a slot in the UR land destruction deck, if it ever returns. Worship in Vain is a plausible way for burn decks to refuel, but as a four-drop that doesn't do anything until turn five, it might be hard to justify. Carve a Path doesn't have enough support in-set to be a viable deck, but in combination with AFM vanishing cards like Savrios' Masterpiece or Doomed Fate, it might create a new archetype. Cold Embrace of Steel is probably not a good card, but I'm looking forward to testing it in my Arbalest Tribal (deathtouch/fight) deck. Ill Fated Travels is unfortunately not actually the best way to cheat a Tarraka-no-Oni into play, but has hilarious potential as an alternative way to get a Liguno. Finally, Alexei might see play in aggressive or midrange decks, but I think it's only really good against other midrange decks, and at 3 toughness is unfortunately fragile.
Sewer Pack supports a dredge-like playstyle that, to my knowledge, hasn't been seen since the MSEM2 reboot. Canal Lurker has potential as a finisher to give G/X control decks some serious inevitability, but its vulnerability to exile removal, like the commonly played Shield's Rebuttal, is rough. At 5 mana, Mob Rallier's probably too expensive for Monogreen Jund, but a similar deck with a higher curve might want it, as might a Naya Tokens deck if one exists. Belligerent's Augur is a lord in the wrong colors, unfortunately, because blue-green Warrior tribal sounds like a beautiful strategy. Coppergild Mercenary is difficult or impossible for UR Spells to deal with permanently, and thus will probably see play as a metagame call against what appears to be MSEM2's current "deck to beat." Fretport Froppers is a key piece in Standard Frog Combo, but with Memory's Collapse not in MSEM, I haven't managed to find a way to rebuild the combo there. Even if it were possible, though, as a three-card combo it's less reliable than existing MSEM classics like Oops All Life. Shared Reminiscence is a solid support piece for a grindy green/X midrange deck, but we haven't seen much in the way of classic Jund or Abzan midrange in MSEM; I'd be curious to see if Shared Reminiscence helps enable such strategies. Fretport's Lament has a lot of potential for helping a mill deck stabilize against aggro, but as unlikely as a good mill deck seems in MSEM, a good green mill deck seems even more farfetched.
Days of Decay is a versatile but inefficient answer; I'm not convinced there's a deck that wants it. All or Nothing is terrifyingly close to Ad Nauseam, and I'm hoping there's an incredibly reckless combo deck that wants it. Madelon's a reasonable sideboard card and an entertaining commander, but I don't think there's a green-blue MSEM2 deck for her to go in. If there is, it probably wants to be running Helene Trant; she's a very good source of value for a controlling or midrangey GU deck. Nataila probably earns a spot in the hypothetical Abzan rock deck I've been mentioning; I don't know if that deck will be good, but I'm looking forward to giving it a spin. Finally, Mass Executions is our first four-mana hard sweeper, strictly speaking—but it's a sweeper for decks that don't want sweepers. I might test a more token-heavy Seto San deck to see if it wants to play a board wipe that refills everyone's hand.
Unjust Sentence is yet another Lava Spike for burn, and the damage is unpreventable to boot. Unfortunately, unlike the other copies of that effect burn's already running, it can't hit planeswalkers. Inciting Bard's playable in a Monogreen Jund deck, probably. Inkwell Daggers is potentially one of the best new cards in the hypothetical Sultai Poisonous deck, but again, that's probably not a good deck. Persecution Official is reminiscent of Meddling Mage; it's a sideboard card, not a maindeck one, but it ought to be good against creature-based combo strategies, at least those that actually cast their creatures. Exhaustive Toil is an incredibly exciting (broken?) combo piece; it generates infinite mana with DYA's new Rime Druid as early as turn 2, as well as being intensely synergistic with PFP's own Plague Mask (see below). Haphazard Militia works as a second copy of Plague Mask in that deck; either allows you to untap your creatures for a mere 1 mana, as often as you like. Finally, Invalidate is a Force Spike and a Mana Tithe at the same time, and I expect it to see play in tempo lists in both blue and white.
Plagues doesn't have much in the way of exciting artifacts, to my view. None of the Equipment seems likely to see play, with the possible exception of Plague Mask, given its easy combo with Exhaustive Toil. Abandoned Memorial lends itself very easily to infinite mana combos with any dork or rock capable of producing three mana, but at six mana itself—requiring a total of eight to combo—I'm skeptical if there's a good shell around it. Artisan's Masterpiece and Arcor Diamond will be casual MSEDH staples, but are both unlikely to see MSEM play. Elixir of Immortality might deserve a sideboard slot if mill ever becomes a serious meta presence, but it's hard to justify over Memorial of the Fallen, which fills the same role in a land-slot. Finally, Discordant Bell is probably unplayable without miracles in the format, but I'm excited to throw together a casual deck using it in combination with TGE's trade mechanic.
Plagues of Fretport brings a new cycle of full-art basic lands, which I expect to see some play but not a whole lot. I personally intend to keep using primarily the Path of Shadows full-art basics.
Much more importantly, it also brings a new full cycle of fetchlands (like Hazy Lagoon), and another of fetchable dual-lands (like Sedate Tundra). I think the tapfetches are generally weaker than the Khaliz-Doran shockfetches. However, they might see play in combination with the TGE cycle-lands, the GNJ sanctums, the TWR utility lands, or any of the new lands from Novea, all of which essentially negate the drawbacks of the tapfetches.
The fetchable painlands, on the other hand, are quite exciting. Much like the K15 checklands, they're best in a manabase heavy on basics and fetches; unlike the K15 checklands, they can provide an untapped dual on the first turn at the tiny cost of a few points of life. Based on what I've seen of them in Standard, the drawback is relatively easy to turn off, especially in a fetch-heavy deck; that said, decks relying on a manabase of shockfetches and fetchable painlands are going to lose a few aggro games they should otherwise have won. Still, I expect to see the fetchable painlands showing up in many 2-3 color aggro and midrange decks.
White: Serene Prayer
Blue: Exquisite Heist
Black: Kalemn Weller
Red: Balanced Blade
Green: Coppergild Mercenary
Multicolor: Exhaustive Toil
Artifact: Plague Mask
Land: The fetchable painland cycle
Command Zone: Madelon, Merchant's Heir