My preview card today isn't really a build-around, but it's the kind of role player with the potential to impact everyone's deck-building decisions. World, meet Ritual of Soot:
Black sweepers have historically been good. Damnation was a Standard staple in its time, and so was Languish. Smaller, more situational sweepers such as Yahenni's Expertise, Consume the Meek, and Infest variants had their moments but were mostly relegated to sideboard play.
At first glance, Ritual of Soot seems to be similar to Consume the Meek, which would give it a "playable" status. I don't think this comparison is apt, though, because one mana means a lot when it comes to sweepers. If your sweeper costs five, then they can often just overrun you before you can even cast it, whereas if it costs four, then they have to play differently merely because it exists, because they will not be able to kill you before that card can be played.
Ritual of Soot has two potential roles, depending on what the format shakes out to be. The first role is that of a Wrath of God—you're just using it because it'll kill most creatures your opponents play. In this regard, the card will play very similarly to Languish. Few people were trying to "combo" Languish with 5-toughness creatures; people were just casting Languish as their best Damnation imitator.
I believe this will be the direction things take with Ritual of Soot. I don't think it's as good as Languish, but I think there's a decent chance it's good enough. My bet is that Goblin Chainwhirler will still be a good card, but the red deck won't be as strong, since a lot of its powerful cards are rotating out. There might be Boros (red-white) or Izzet (blue-red) aggro decks, but those might just not be able to support casting a triple-red card early on in the game. If this is the case, then certain strategies that have been kept in check by Chainwhirler may flourish again, and then Ritual of Soot will be good against those.
Ritual of Soot seems like it'll be useful against every aggro deck (especially now that red aggro decks will lose some of their late game power in Chandra, Torch of Defiance; Hazoret the Fervent; and Glorybringer), but it seems particularly great versus white-based aggro decks. There are a lot of powerful token strategies in white already (History of Benalia, Benalish Marshal, Legion's Landing), and Ritual of Soot looks like a clean answer to those archetypes. Being able to answer their two-drop and their History of Benalia with only one card is very appealing.
There's also the fact that Selesnya (green-white) is a guild that's featured in Guilds of Ravnica. Selesnya has historically had a tokens theme, and we can see that coming back in cards like Emmara, Soul of the Accord (which you can see in the Card Image Gallery, if you haven't checked it out yet!). The guild's mechanic, convoke, also leans itself to a token-centric theme, or at least a swarm theme. Those decks should be vulnerable to Ritual of Soot and will have to keep it in mind during both playing and deck building.
I also think Ritual of Soot might become even better with the set after Guilds of Ravnica. Right now, in Standard, we have Merfolk and Vampire tribes, which have a ton of ways to pump their own creatures. Ravnica Allegiance will feature both Simic (green-blue) and Orzhov (white-black), so we might see tribal decks in those combinations. Tribal decks tend to have ways of pumping their creatures outside the range of a card like Yaheeni's Expertise, but Ritual of Soot doesn't care about that.
The potential role for Ritual of Soot is to synergize with your own creatures that specifically don't die to it. The curve is a bit awkward, since you can never curve a three-drop into a Ritual of Soot, but it's possible to cast a four-drop (say, Nicol Bolas, the Ravager) and then on turn five cast Ritual of Soot. I don't think this is going to be a main reason for playing the card, but it is good synergy if it lines up, so you should keep it in mind. If you're playing Ritual of Soot, you can control what it's hitting from your own deck to some extent, so it's not as symmetric as it might appear at first glance.
All in all, I think most control decks that have access to black mana will want Ritual of Soot. Whether it's a main deck card that shapes the format or a sideboard card that you bring in against certain archetypes will depend entirely on what threats are being played, but I expect it to fit somewhere between those two classifications.