Ari mentioned in his last article that Jeskai Ascendancy has some potential as a combo card in Modern, and I think it could turn out to be one of the best combo decks in the format. It essentially turns every mana creature into a Goblin Electromancer that can give spells zero or negative casting costs, which would be pretty good by itself, but it also loots, meaning you basically never fizzle, and it doubles as a win condition by pumping your creatures. Merely translating the text of the card, I kind of wonder how it was ever printed, and that's before even getting to a decklist that exploits the nuances.
Ranjan Pradeep posted a build on Facebook that a lot of ideas I really liked, but a couple that I didn't. Based on his list, I'd start with:
This deck can kill on turn 2. All you need is a mana creature, a Jeskai Ascendancy, and a Gitaxian Probe, that gives you one mana with Ascendancy in play, from there, things are still a bit tricky, but you can now cast any number of Gitaxian Probes, Cerulean Wisps, Sleight of Hands, and Serum Visions for free, and each one comes with an extra loot to find another, discarding any creatures or lands you draw. If you cast enough of those to get seven cards in your graveyard, Treasure Cruise also becomes free. A second Gitaxian Probe or a Cerulean Wisps nets you a mana at this point, getting you up to two, where you can now cast Manamorphose, which will also net you a mana. Your goal is to get up to four mana so that you can cast a second Ascendancy and another card draw spell, and then each spell will untap your creature twice, which means you net mana on basically everything you do, and very quickly, if not even before you were able to play the second Jeskai Ascendancy, you'll be able to just attack for 20. If your opponent played a blocker, you can Glittering Wish for Abrupt Decay if you have access to black mana, or Simic Charm if you have a Noble Hierarch to clear the way first.
That required a lot of the right cards, but not an unrealistic number, and that's just for the turn 2 kill. If you have an extra turn, you can do things like play a second mana creature, which makes going off far easier, or Glittering Wish for your Jeskai Ascendancy.
On top of having an insane base combo, this deck also gets to optimally use what might quietly be the best broadly applicable Khans of Tarkir card in Modern: Treasure Cruise. Treasure Cruise has some serious competition here, in particular from its sister card, Dig Though Time, as both cards are absolutely absurd, but Treasure Cruise is the way to go here since you really want to be able to spend just a single blue mana to cast it.
Treasure Cruise is another messed up card--one that I think could easily prove to be a vastly superior Ancestral Visions, since it similarly gives you a huge bump in the midgame, but it doesn't require that you spend your first turn on it, and it works as soon as you topdeck it later in the game. One problem with it is its raw power level, and the fact that it often wants to go in the same kinds of decks as Snapcaster Mage, and that just means people start to have Relic of Progenitus against you a lot more, which can make it hard to start your cruise. Even when that happens, the card isn't strictly dead, since you can often outrace Relic of Progenitus to some degree, and it's not the end of the world if you have to spend a reasonable amount of mana to draw three cards some portion of the time.
In this deck, Treasure Cruise is particularly good because you're not doing anything else with the graveyard, so there's no real cost to exiling cards, and it's hard for the opponent to justify bringing in hate.
Decks with a lot of removal might make it hard to go off, which is why I suggest playing four Sylvan Caryatids, though Ranjan had two in the main and two in the sideboard, which could also work. If you go off with a Sylvan Caryatid, you won't be able to attack, which is why the deck plays a Grapeshot.
Dryad Arbor makes it much easier to get two creatures in play so that you're netting mana on your plays, since you can fetch it at the end of your opponent's turn before going off. The problem is that it only makes green mana, which is pretty useless without Manamorphose, but it can also give you an attacker if you have Sylvan Caryatid.
Cerulean Wisps is the card that looks weirdest to see in a deck at first, but it's pretty clearly awesome here, since it nets you a mana when you're trying to go off, and can cycle for free before that.
So, the deck is clearly very fast, that is indisputable with it's good draws. I think it's very consistent, since it's all card filtering and Glittering Wish just gets you what you need immediately, and Jeskai Ascendancy makes fizzling extremely unlikely once you get started. The only other question is how easy it is to hate out. You have to have a creature, but you have a lot of copies, it can be any creature, and they all either cost one mana or have hexproof and require no additional investment that would get you two for one'd outside of an occasional Cerulean Wisps.
This basically feels like Splinter Twin, except your Deceiver Exarch is a literal Birds of Paradise, and your Splinter Twin costs three mana, doesn't care if your Birds of Paradise dies in response, and has an incredibly powerful passive ability even when you're not going off.
I'm not trying to oversell this or create unnecessary hype, but how is this a thing?
Anyway, hate cards: graveyard hate doesn't matter, Leyline of Sanctity doesn't matter. Creature removal is good, but nowhere nearly as good as it is against basically any other combo deck that uses creatures. Enchantment removal is good, but no better than it is against Twin. Discard is good, but it will be against every combo deck. The really great cards against this deck are the storm hate cards--Rule of Law, Ethersworn Canonist, Eidolon of the Great Revel, Eidolon of Rhetoric, Spirit of the Labyrinth, and the like. Glittering Wish gives you access to answers to those cards, but they're still real problems. One side effect of printing Jeskai Ascendancy might be to push traditional Storm out of the format just by forcing people to play more of those (and also possibly being a more or less strict upgrade of that deck).
The deck looks so close to a Storm deck that I wonder if it would be possible to just play four copies of Jeskai Ascendancy, cut the Glittering Wish, and play Pyromancer Ascension as a backup win condition that doesn't require a creatures, but I'm pretty sure the creatures would get in the way if you didn't have the looting from Jeskai Ascendancy anyway, and Glittering Wish also gives you incidental access to a nice toolbox (which I almost certainly haven't tuned perfectly).
If you wanted to make a hybrid Jeskai Ascendancy/Pyromancer Ascension deck, I think the key would be cutting down to 8-10 mana creatures and replacing them with more cantrips which probably wouldn't be a problem, since you could still draw into them and you'd be able to go off without them potentially--here, the problem is that without rituals other than Manamorphose it's hard to really go off with Pyromancer Ascension, so likely you'd just be using it to set up Jeskai Ascendancy. I think that what this means is that this kind of change would make the deck slower, as cutting mana accelerants decreases your nut draws, and Pyromancer Ascension takes time to set up. It is a little more resistant, especially against control decks, where Pyromancer Ascension has always been good.
Space is really tight in the deck, since you need to play a reasonable number of mana accelerants (though you could maybe cut back by 2-4 of those depending on what you're replacing them with), and most of the cantrips are just the clear best versions available, however, there are still some other cards to consider.
Crimson Wisps lets you go off from an empty board by playing a creature and giving it haste to get started, and it also lets you get a second creature going mid-combo to start making extra mana. I could see this as either a one-of or a four-of depending on whether you're trying to use it just to get extra mana going once you're already going off or whether you're trying to plan your game around often going off with a hasted creature. I suspect one is more likely, but I don't know if it's worth it.
Another similar option is playing Twinflame, which accomplishes the same goal without you needing to actually play a second creature, it just gives you another hasted creature that can start making mana right away. This would likely help your turn 2 win percentage, but it's absolutely useless when you're not going off and doesn't cycle, so it probably makes the deck worse overall--you could cut the Grapeshot for it though, as then you're not changing the number of cards that work that way, and this helps you get your combo going, unlike Grapeshot. However, like Grapeshot, it allows you to win if your only creature is Sylvan Caryatid, as you just play any other creature, then Twinflame it, and then make that copy huge. In fact, I would guess this is better than Grapeshot, except that Grapeshot also lets you kill random creatures early, especially their mana accelerants in the mirror match.
Another alternative to Grapeshot would be to just use that space on something else, like a fourth Treasure Cruise, and add Flesh // Blood to the sideboard, which would let you win with a Sylvan Caryatid or through any number of blockers, or Ensnaring Bridge or whatever else. Again, this is likely preferable to actually playing Grapeshot.
The manabase is also not set in stone--I'm not at all confident that Dryad Arbor is worth it, as it could easily just be too cute and too terrible to actually draw, and this might be too many lands that don't tap for blue. It's possible the manabase should be more like 4 Gemstone Mine, 4 City of Brass, 4 Mana Confluence, 4 Breeding Pool or something, but that also makes Treasure Cruise a lot worse, as you're not playing fetches, and that mana will hurt you a lot if the game has to go long. I think I'd rather stick with the mana closer to how I have it listed, where the only problem might be getting black if you want to wish for a black card, but it's very rare that you'll have to before you're going off, and then you can just Manamorphose for the black.
All in all, I think this might just be the premier combo deck in Modern in the next couple weeks, and it's certainly worth your time to at least try it out a bit. Getting the exact right number of mana accelerants and finding the exact bullets, especially for Glittering Wish, might take some time, but this shell really exploits what might be the most broken card printed in years.