Mythicspoiler.com has over half the cards in M15 listed already, which is a surprisingly large amount of information this far out as far as I'm concerned.
As we know, Magic is a game about interactions more than it's a game about cards, so for my first look at M15 I thought I'd go through and talk about what other cards I associate with the new cards and what interactions I see as potentially significant for these. This is going to be very freeform, and free association, as if you were to name a card and I'd say the first thing that pops into my head.
First up, the Soul of _____ cycle. These are definitely powerful finishers, but you need to untap with them to take advantage of their ability, and then using it takes up most of your turn. These aren't the free bonus value guaranteed up-front that we saw with the Titans, and as spells to cast, they're a lot weaker. However, the most important part of these cards is obviously that they do something when they're countered, milled, or you have to discard them. This gives your expensive spell some resistance to Thoughtseize, and gives your graveyard deck even more late-game value.
The obvious card to think of with these would be something like Satyr Wayfinder - a fine card to play for value that gets a lot better when you can hit random other relevant cards in addition to the land. For me, though, the main card I think of is Scavenging Ooze. I think these cards will see some play, but I think that for now Scavenging Ooze will gain popularity and do a really good job of keeping them in check... and the harder people work for them, the more Scavenging Ooze will punish them.
Another card that gains value here is the already-underappreciated Silence the Believers, which also makes casting these a pretty terrible endgame plan. I think this cycle is significant, and some or all of them will see play, but I don't think we're going to see the format warp around them the way we did with Titans.
This card is pretty crazy, by which I mostly mean hard to evaluate. It seems incredible in Limited. For Constructed, it's really one of the worst things you can try to do to help your white weenie deck's goldfishing clock, which means it's only good against people who are trying to interact with you, but that's not a bad thing - most people are trying to interact with you. This card is pretty great against Black Devotion decks, since every threat comes with another threat, making it such that trying to keep your guys in check with spot removal is a losing proposition. I think that means this is at least worth serious sideboard consideration.
As for interactions with other cards, I think we'll primary see this in decks that are just playing it with random creatures and not trying to particularly abuse it, but if you wanted to build around it you certainly can. Creatures that flicker like Obzedat, Ghost Council and Aetherling trigger it every time they enter the battlefield, but that's obviously going way too big. A more realistic plan would be to combine it with Deputy of Acquittals and/or Quickling, another card I wanted to talk about separately.
In Modern, there's really nothing I would like more than finding out that this card was somehow the missing piece for Faeries, and that returning my Spellstutter Sprite to my hand to buy another counterspell in response to them using a removal spell on it (because using a removal spell on a Spellstutter Sprite into 1U untapped is a totally reasonable play that people make all the time?) is going to be a play I can regularly make. Realistically, if Faeries is good enough, this might be a card you can play one or two copies of, but I wouldn't get my hope up too much for this kind of thing.
This card reads as quite good to me. Flying is a pretty big perk for a Whitemane Lion, even if it's worse to have to sacrifice this than bounce it if they kill my other creature in response. Historically, though, these cards tend to underperform, and Deputy of Acquittals doesn't really see play.
I'd love to believe that some combination of Quickling, Deputy of Acquittals. Spirit Bonds, and Ephara, God of the Polis can form the core of a reasonable deck, but aside from the premise being somewhat fundamentally too cute, I don't really think we have enough cheap creatures that we're independently getting value from returning. This deck would absolutely love something as weak as Cathedral Sanctifier, but there isn't a one-mana creature we can generate value off of. At two, we get Omenspeaker or Azorius Arrester, and at three we have some options, but they're weak - Seller of Songbirds isn't a card I want to build my Standard deck around, especially when I already have Spirit Bonds. On the other hand, M15 does offer Frost Lynx, and Kor Hookmaster was a great Limited card and fringe Constructed playable. Another piece of the puzzle might be Illusory Angel, which benefits from the fact that you want to play a lot of cheap creatures and always have one in your hand because you're returning them so much.
Overall, this deck sounds cute, but likely not good enough.
This card is serious. I'm not saying it's good, that it will be format-defining, high impact, or even playable, but it's certainly doing something unusual. The primary options for taking advantage of this card are to play it in decks with extremely low curves that are happy to play it when they have four or five lands in play and think of it as a way to draw four or six cards or to build an engine around mitigating its drawback, but that's quite hard to do.
You can obviously use cards like Rampant Growth to get more lands into play, but sacrificing a land to draw two and then drawing into more lands that you can't play isn't particularly useful. It's best if you have a plan for the lands that are clogging up your hand. Maybe Zombie Infestation, but I'd hope we could do better. Seismic Assault is at least another card that our deck that doesn't want many lands in play could take advantage of independently as well.
Interestingly, while cards that allow you to play an extra land don't help with this at all, cards like Sakura-Tribe Scout do allow you to get around the drawback. At first, I didn't notice the “once per turn” and I wondered if this could do something sweet in Legacy with Manabond. I suppose that combo still gets you somewhere, especially since you can use it on your turn and theirs, and drawing four extra cards a turn certainly isn't trivial, but it's not clear what the rest of that deck would look like.
It's an interesting puzzle. This is the kind of card that I don't expect to ever see serious play, but I wouldn't be shocked to learn someone had broken it.
It's pretty weird to see these cards here. I don't feel like Standard really supports them, and they already exist in Modern, so what's the point? It makes me wonder if artifacts will feature prominently in Khans of Tarkir. In the meantime, I don't like Shrapnel Blast's chances, but Darksteel Citadel is a pretty great tool for Trading Post so there's some chance it'll cause a minor resurgence in Trading Post Control decks in Standard, which I think would be pretty sweet.
This is a pretty innocuous card that I periodically find myself wishing I had access to. It's obviously good with things like Spear of Heliod that pump all your creatures, and the fact that it's an instant helps against sweepers in particular, but it's the presence of Convoke in the set that really makes me think this card has a chance.
This seems great to me. The list of cards it kills is going to be terribly obvious, but I'm obligated to mention the highlights: Courser of Kruphix, Chained to the Rocks, Banishing Light, Detention Sphere, Underworld Connections, Eidolon of the Great Revel. The list goes on and on, of course, and the point is that most decks have things for this to kill. This card seems hugely significant to me, and if Khans of Tarkir does have an artifact theme it only gets better. The question is, which decks can best use this, how many is it realistic to play in a maindeck, and what does that do to deckbuilding for other decks? Will this cause a chain reaction that sees a decrease in all those enchantments? What do control decks do about planeswalkers and enchantments when they can't rely on those? Which decks get better because Banishing Light and Detention Sphere are holding them back less? Maybe a 2/1 body just isn't enough to matter, but merely putting this on a creature means that things like returning it with Whip of Erebos for more disenchants start happening. This card feels very real to me.
Is there any chance this replaces Bident of Thassa in Mono-Blue Devotion? It certainly doesn't draw as many cards, but do you really need to? If the point is “establish a card advantage engine,” this can do that for half the price, and the blue deck can really only afford so many four-mana spells... and there's a lot of competition. Plus this comes down early enough to dodge counterspells, and with a low curve it's easy to start drawing on turn three. The primary problem with this card is that it doesn't play well with my preferred style of play against control decks, which is to simply play one creature at a time and make them deal with each individually. This forces you to pay them at least a little for their Supreme Verdicts. Still, there's definitely enough upside to make this worth investigating.
Also, this costs only a single blue, so the applications certainly aren't limited to Mono-Blue Devotion. This card is great with Raise the Alarm, and with White Weenie strategies in general.
It's really awkward that this card doesn't work with Sin Collector or Lifebane Zombie. In Standard, this is making your Duresses draw a card, and your Thoughtseizes draw a card or make a 2/2. The only way to really go big with this card is Whispering Madness, which I fully endorse. I'm not sure exactly what happens when you start pairing this with Whispering Madness on a creature you can connect with, but I have a feeling it gets pretty great.
In Modern, the best thing I can think of off the top of my head is Burning Inquiry, which is a card that has already seen a little play in black decks with graveyard interactions.
My inclination is to think of this card as a Megrim variant, which I don't think of as a strong pedigree. How much better can this card really be than Liliana's Caress? Though I suppose another question is: once we can pair this with Liliana's Caress, do we have enough payoff to go far enough out of our way to play things like Burning Inquiry?
This card is sweet. I'd be surprised if it's good enough for Constructed, sadly, but I love it as a Limited card to enable a splash, and something about the flavor just makes me really happy - ”I'm going to kill you by dropping a giant rock on you. Oh, by the way, I can also get mana from this rock.”
This card confuses me. I don't understand putting it in the set if there aren't any other Slivers, and we've seen over half the set without seeing any. I assume this is just because they wanted to wait to reveal them until after the Sliver Hive, but maybe it's just a random one-of Sliver card to make players curious. We've seen it before with Eye of Ugin.
If this is the only Sliver card, Slivers still won't matter in Standard, but if there's something like one common, uncommon, and rare Sliver in each color, I'd definitely want to test them. Mana Confluence, Sliver Hive, and Manaweft Sliver go a really long way toward making the mana work.
While I'd like to see Slivers in Standard just because I don't like seeing such a large section of cards never get used, I think it's actually unlikely that there are other Slivers in the set, just because I wouldn't really want Slivers to be a thing in two core set limited formats in a row - it would just be kind of boring, and my guess, which could be known to be wrong at the time you're reading this, awkwardly, is that they're not doing that.
So, the next question is, does this make Slivers a thing in Modern? We have Sliver Hive, Cavern of Souls, Ancient Ziggurat if we want it, Reflecting Pool, City of Brass, Mana Confluence, Gemstone Mine, and others... so we can easily get all the colors. It's more likely that the mana doesn't need to be that good, and we can afford to play fewer lands with drawbacks. Unfortunately, I don't think mana was the problem with Slivers in Modern. The problem is that there's a lot of good removal, so you can't really get them working together, and they don't usually race fast combo. It's not really clear what deck you'd be trying to beat by playing Slivers. If we had Crystalline Sliver, we could talk.
Those are the cards that really spoke to me so far, but there's still a lot to see. So far, M15 feels a little haphazard with the random guest designers, cards that look out of place like Sliver Hive and Darksteel Citadel, and from what we've seen the complexity level of the cards (just in terms of number of words) looks staggeringly high for a core set. Hopefully that's just because we've seen a disproportionate number of rares. A lot of the guest designed cards are pretty sweet, but I think in total it just looks like too many cards that are trying too hard and clamoring for attention without any real cohesion, but I'd love to be proven wrong. It might actually be pretty cool if we learned that it's good for sets to just have a lot of independently sweet cards.