Modern's playable card pool is the largest of any format.
This is a fact.
While the overall card pool isn't nearly as large as Legacy or Vintage, the amount of cards that can, or better yet, have done well in a large event vastly outnumber the other two Eternal formats. The range of decks you can actually play and not feel bad about are well within the twenties. I don't think you can say the same for almost any other format.
Whether that makes for a "good" format for whatever setting is not the point. The point is that because of how vast the range of decks are, the amount of room for further implementation of sub-strategies within these decks is pretty large. I think that there are a lot of cards that help enable this sort of thing to happen but have a hard time making their way into Modern because of the fixation that you have to try and break the format every time a tournament comes around. That art is dying. You are rewarded for continually progressing what deck you have a natural affinity or preference for, and that won't change for a while.
With that said, I have a pretty decent amount of cards that I think might have a chance at being a potential tool for said progression.
Thalia is a staple in the G/W Hate Bears deck and can be spotted in a bunch of other assorted aggressive decks, and even a few midrange decks here and there, so why would this be on the list?
Well, it's still unbelievably underplayed.
Part of it is because a large portion of the white decks are underpowered despite boasting some of the strongest sideboard cards. Path to Exile is probably the most misutilized and misevaluated card in Modern, but is jammed as an infinite-of anyway. Thalia can completely cripple a heck of a lot of strategies, especially a lot of unfair ones, but it isn't viewed this way for Mogis knows why. I think that the fear of things like Splinter Twin and just dying to aggressive creatures is what makes Thalia's stock lower, but I have a strong feeling that these creature decks are going to want to just stymy them just enough to get their offensive line going, and Thalia is just the creature for the job. What you don't want to be doing is jamming this in your Collected Company or burn-heavy Zoo decks. I would generally want to have this in some sort of bigger Zoo deck that has a lot of creature-spells and super cheap interaction that's kept to a minimum.
As crazy as it sounds, I don't think that Bring to Light will be the card that brings Scapeshift over the top, though it does help immensely. Commune with Lava does a whole lot of what Scapeshift wants to do already, which is to see as many cards as possible and interact at very specific points with options at its disposal. Commune with Lava is the bridge to that gap, making that transition very easy, especially when you have to lean on a very specific subset of things happening to get that Scapeshift or Bring to Light through. If Bring to Light is Scapeshift five through seven or eight, then Commune with Lava is Bring to Light five through whatever. I would hesitate to put this in any deck that can't generate a lot of mana or can't convert the batch of cards you'd get into a win on the spot. So a deck like Grixis Control or Jeskai wouldn't want them (Jeskai has Sphinx's Revelation, and Grixis has a card on this list).
Now, I'll be the first to tell you that I am just not a big fan of this card, and I never have been. With that, I know a huge payoff card when I see one, and I think Burn decks and other assorted tempo decks could make pretty nice usage out of it.
I bet that you didn't think I'd mention it in tandem with Burn.
An unanswered Monastery Mentor can actually prove more potent than many creatures in the sideboard, including Kor Firewalker. It's not that the card is more powerful than Kor Firewalker against the mirror and other related decks, but it does provide enough overlap where you'd jam it against them anyway. It's also just a powerful creature against decks that are strained on hitting their colors as painlessly as possible while also having their removal line up. If you're on a creature-heavy draw, then the likelihood of Mentor surviving the earlier turns is higher; and if you're on a land-heavy draw, then you gain some momentum there too. This dimension of snowballing things out of control out of your linear combo-aggro deck is very unexplored, and is one of the more interesting places to try it. I wouldn't try this in any sort of heavy control deck unless the amount of cheap interaction you have is pumped to the brim. Even then, it would take a lot for me to play it in a very slow deck over some of the more robust and straightforward finishers out there.
While Dispel has been one of the most popular sideboard cards in the better half of the year, I firmly believe that it's time to bench it and slant heavier toward having general non-creature answers to things. The decks that play blue in Modern very likely don't have many clean answers to enchantments or sorceries, and if they do, it's any combination of slow, expensive, and narrow. Spell Pierce is the cheapest way to make sure you hit what's important, regardless of what it is. From Hive Mind, to Boros Charm and Rift Bolt, to Blood Moon, to Cranial Plating. The obvious weakness of Spell Pierce isn't as pronounced at this stage in Modern because most players are prone to jamming nowadays, especially in the face of a single blue. That Splinter Twin player will jam it seven out of ten times against your Spell Pierce because the upside is so high for them. I usually don't advocate the stock of a card being higher due to the behavioral patterns and trends of players, but this one is too good to pass up.
Liliana of the Veil will always be the premier planeswalker of Modern unless something insane happens. This doesn't mean that Ashiok doesn't have a place. I think that Ashiok is an excellent role player against midrange decks after sideboard, mostly because of how quickly they can get the ball rolling and potentially pseudo "ramp" you into a more expensive creature. Investing three mana to have the potential to add two things to the battlefield in one turn on the following turn is insane, and something that Liliana couldn't ever do. While I don't think it's an easy inclusion, it's definitely a solid card if you're looking to have a cheap way of creating potential midrange gold on lay-away. It would not surprise me to see more Ashiok outside of "Gerard Fabiano's box of cards.dec" decks within the next three months.
I was one of many people who brushed this off as borderline, perhaps unplayable, especially in Modern. The format is painful enough, and investing three mana into a card that could get you killed so easily is not something I was very fond of.
But a lot of that simply isn't true.
The format isn't nearly as painful as it once was, especially if you're already in the market to play a card like Painful Truths. Often times, you're okay with taking a small hit in mana consistency if it means saving two life, and that's two life that would keep you in the game. Fetching basics first has been the norm if you could afford to, and even if you couldn't, you probably still did. The other mistake I made when evaluating this card was making a constant comparison to Read the Bones, which brought out how much I overrated scrying relative to cost. Basically, the cheaper it is to scry, the more valuable it becomes. When we get to three mana, I'd much rather have the card than a scry 2.
The tension with the card actually isn't with how much damage you're taking off of your lands, it's how well you can keep yourself from dying after resolving this. Of course, decks like Burn and Zoo have the potential to punish you pretty hard for casting it, but the three-color decks could use a bit more torque, and this card is likely the best at providing just that.
You may have not expected a lot of these cards to make the list, and there are a lot of cards that are certainly worth mentioning, like Ulamog, the Ceaseless Hunger in G/R Tron or even something as innocuous as Anticipate in Ad Nauseam. The range of cards that could pop up and help you out in a tournament is so wide. One of the things I do is have a list of cards after searching through Gatherer, even the most corner case card ever. If it's even remotely worth something in the tightest of situations, I have it on me anyway. You never know when that corner case becomes an uncommon case, or when that uncommon case becomes an actual problem. It's better to have it and consider it and not need it later than to never know about it and miss it when it could've been in your range.
What cards do you believe are worth considering for some decks in Modern?