I can't recall a set as exciting as Modern Horizons.
While this isn't labeled as a Masters set, I imagine the power level across the board will be similar enough that we can use what we know from prior environments to speculate on this set. Masters Limited formats are my favorite, and adding the flexibility of new cards means the potential for this Limited format is through the roof.
Nothing has been announced regarding Limited archetypes, but everybody is already speculating on what will or won't be in the set. The way I see it, a card will be included in Modern Horizons as a common or uncommon either for the Limited format or for Modern. Designing the set to maximize the number of cards that are for both Limited and Modern would yield the best set.
Starting easy, I expect there to be land cycles. At least one, but maybe two. While Ultimate Masters didn't have one, they're fairly commonplace in all Limited formats. I looked at all the lands that are not Modern-legal and two cycles stood out:
Cycling is one of the best mechanics for Limited and a perfect mechanic to bring back for this set, as plenty of old cards with cycling could be good tools to add to Modern. Cycling lands are one of the reasons that Hour of Devastation was such a fantastic Limited format and these are even better!
And if they decide to include cycling lands, and maybe even go for a cycling theme, I would expect both Astral Slide and Lightning Rift to be in the set. While I don't think they'd likely have a huge impact on Modern, they're fan favorites and sweet build-around cards!
The other cycle of lands that would make for an interesting Limited format: the threshold lands from Odyssey. They give a directional theme to the set — the graveyard — and are powerful for Limited. The only problem I see regarding their inclusion is that Nomad Stadium is mostly worse than a Plains, and Cephalid Coliseum may be too good for Modern. However, if Wizards of the Coast wants to put Nimble Mongoose into Modern, maybe this cycle of lands is the perfect fit.
I'm guessing that we'll either get the cycle lands or a set of new multicolor lands. Maybe both! Threshold was already a theme in Eternal Masters and Ultimate Masters had a very strong graveyard focus.
As far as other lands that could be included in the set at common or uncommon, I think either Mishra's Factory or Crystal Vein is a good bet, with a much higher likelihood of Mishra's Factory. Crystal Vein would be another tool like Simian Spirit Guide, though it punishes you more if you use it in the early turns since it costs a land drop. Mishra's Factory is a great card for Limited and could be a solid inclusion in Modern decks like Affinity and Hardened Scales.
I think it makes a lot of sense for cycling to be a core mechanic in this set. So what are some cards with cycling that might do Modern good?
Those are the only cards with cycling currently not in Modern that I believe would see a reasonable amount of play. And no, I'm not considering Cloud of Faeries. Nobody does anything good with that card. Let's not put that into Modern.
This doesn't mean that cycling can't be a theme of Modern Horizons. The cycling lands can play an important role and new cards can be put in the set for that purpose. Cards like the Chartooth Cougar cycle, Expunge, Complicate, and more could be great for the Limited format. And I still think it's likely for cycling to play a role in the set. But it's not because I think Miscalculation and Unearth need to be in Modern.
Emma Handy mentioned recently that tribal cards are likely on the list of things that Wizards of the Coast wants to buff. She cites Slivers, Goblins, and Elves as the most likely, which is reasonable, although Zombies, Soldiers, and even Wizards could get some support as well. A tribal Limited environment isn't easy to support without dedicating too many slots to the cards, which can make Draft somewhat anemic. But given that there's a cycle of tribal cards with cycling, I imagine that the support for these could go hand in hand (especially given that Gempalm Incinerator is good enough to see Modern play).
Slivers, on the other hand, are very unlikely to end up in the set. Old Slivers affect every Sliver on the battlefield, including your opponent's. This creates gameplay that Wizards of the Coast tends to avoid. Maybe it would be okay for Modern, but it would be a bit much for Limited. Additionally, the whole environment would have to be warped around Slivers. Unlike Goblins and Elves, Slivers exist across all colors and need a lot more support. They're rarely playable without a density of Slivers, while many Goblin and Elf cards can be fine inclusions in any red and green deck. I can't see Wizards of the Coast jumping through hoops to reprint old Slivers and make new ones as well.
The only way I could see the set going full-blown tribal in every color with the addition of Slivers is if the set has a bunch of new changelings to tie it all together. I would hedge my bets towards just Elves and Goblins.
Without much more information on the set, there's only one more space to look at in regards to Limited: the instant and sorcery spells. The creatures in the set will be contingent on the archetypes. Will there be a tribal theme? Will Rakdos have a sacrifice theme as it usually does? But the card draw and removal spells aren't contingent on the themes.
I would love to see blue get awesome cheap interaction like Counterspell. It's the perfect addition to the set and we've had it at common in Masters Limited formats before!
This is where the reprints start to get dicey. Swords to Plowshares may be too powerful or too much of a color pie bend to be put in the set, according to the Council of Colors. Unfortunately there aren't other pieces of white interaction that I can think of that would make waves in Modern.
I think Flametongue Kavu is pretty much a shoo-in for the set. It's a huge part of Magic history, and is a powerful Limited card. It's unlikely to do much in Modern but is a great reprint for the set. Firebolt and Goblin Bombardment would be wonderful additions to Modern and I think they're likely to be in the set. The real question for Goblin Bombardment is if it can be at uncommon or if that breaks the Limited set and has to be bumped to rare. And I don't think they'd want to use a precious rare slot on Goblin Bombardment.
Black is king of removal, and it's not surprising that there's an abundance of potential reprints for the set. I imagine either Diabolic Edict or Chainer's Edict will make it into the set, but not both. Two Edicts is a lot for Limited, especially since they don't tend to be very good.
I would love to see Snuff Out in Modern. While zero-mana removal could be argued as too good, four life is a lot to pay in a world with shocklands and it would be a fantastic card to throw into Death's Shadow!
Bone Shredder and Expunge would be nice cards for the Limited environment, but they're likely too slow for Modern. However, if there's a cycling theme, I expect Expunge to be in the set. If threshold is a mechanic that matters, Bone Shredder could be a nice addition as an abusable removal spell that, if you want, does eventually end up in the graveyard. And if that's a theme, Ghastly Demise is the perfect card, and may be efficient enough to end up in Modern!
Last, Malicious Affliction is an extremely powerful card that will be in the set under one condition: Aristocrats is a supported archetype in either Rakdos or Orzhov. It's important to seed a set with good cards that are great in specific archetypes, and Malicious Affliction checks all the boxes, including being powerful enough to see Modern play!
I could keep going with speculation on speculation, but I've exhausted what I could reasonably predict based on the information available. Modern Horizons is an extremely exciting set and I can't wait to see how it turns out!
What do you think will be in the set for the purposes of Limited? Anything I missed?