While Guilds of Ravnica doesn't feature anything for Jeskai as impactful as Teferi, there are a few new options to consider. Besides, the Modern metagame has evolved enough over the past few months, it's worth taking a deeper look into some of the updates people have had success with.
The biggest departure from the previous world has been the widespread shift towards Terminus, as the sweeper of choice. Jace, the Mind Sculptor combos with it for some "Miracles lite" action, and Opt really helps get it over the finish line (making it more likely that you'll get to miracle it on your opponent's turn)
That's only part of the story, however. A big part of the reason for Terminus gaining so much popularity is the shift in opposing strategies.
Humans has established itself as a tier 1 strategy, now that Unclaimed Territory stacks with its previous fixers to give it basically perfect mana, all of the time. Besides, do you want Supreme Verdict or Terminus against stuff like Christopher Slinger's Tron deck?
With a host of durable threats like Wurmcoil Engine, Worldbreaker, and Ulamog, the Ceaseless Hunger, Slinger's list is a perfect example of where Supreme Verdict fails and Terminus succeeds, on account of putting the creatures on the bottom of their deck, instead of actually killing them.
For an example of this new-ish Terminus style of Jeskai, let's look at IQ winner Henry Holbrook's most recent list:
This is a reasonably standard build of the archetype, even including the split between Lightning Bolt and Lightning Helix. While Lightning Bolt is the objectively stronger card, drawing a Helix against aggressive decks really increases the value of your Snapcaster Mages.
If you've got to pay two life to get the extra mana from a Steam Vents, you're still netting a life compared to playing it tapped and Bolting the threat. Besides, Teferi untapping two lands means you're going to have exactly two mana a lot of the time anyway, so you might as well put it to good use.
Most people that use Terminus at all, go hard on it, and I guess I'm not so sure it ought to be so all-or-nothing. Drawing two Terminus is generally way worse than one plus one of either Settle the Wreckage or Supreme Verdict. Would it be crazy to play two Terminus, one Settle, and one Verdict? At the very least, I'd be interested in trying three Terminus and one cheaper sweeper.
I still think Temple of Epiphany is slightly underrated, as the extra card filtering really is quite similar to Opt, and if you were going to have to play your Sulfur Falls tapped anyway, you're kind of freerolling. Once you're willing to play a Lightning Helix over another Lightning Bolt, there's at least some willingness to sacrifice guaranteed speed.
Holbrook doesn't play Secure the Wastes or Sphinx's Revelation, unlike some Jeskai pilots that play one or the other as a high-end alternate victory condition/way to take over the game, and I don't blame him.
Having both Teferi and Jace means we've got a little more redundancy in kill (of course, along with Snapcaster Mage and Celestial Colonnade), but it's still quite valuable to have another threat, another way to get the game over with before things spiral out of control. In this slot, Holbrook runs Vendilion Clique, instead. Clique is a great card, so I can't fault him, but I'm interested in trying Expansion//Explosion instead.
Expansion//Explosion has been a Standard staple since Guilds of Ravnica first dropped, but like Sphinx's Revelation, it's the sort of card that will likely cross over once people figure out the best way to utilize it. A Sphinx's Revelation that costs a mana more in order to deal damage instead of gaining life isn't particularly better; but the ability to "cycle" it early as a Fork/Twincast is actually really interesting.
There are some nice opposing cards to copy and you can generally use it as a counterspell against counterspells; however, there are actually quite a few decks in Modern without any good targets so I think you really need to believe in the power of copying your own stuff, like Lightning Bolt, Lightning Helix, and the like.
Planning to copy your own Ancestral Vision is probably just "win-more," but it's a good card to keep in mind from opponents. You can copy their Vision(s), and then if you draw a counterspell, copy the original.
I know a lot of people like to sideboard Ancestral Vision against Golgari, Jund, and the like. I've been on that side plenty of times, but don't love it, right now and prefer Holbrook's use of zero. It's just such a bad topdeck.
While the above list features zero Hieroglyphic Illumination, I don't mind one. It's only a little weaker than Opt to cycle, and having the option of one of these, as the game stretches on, is kind of nice for grindy matchups where you'd normally be interested in Ancestral Vision.
I think Ral is definitely above the line for Modern playable, and some Jeskai decks use one in place of a Teferi. Still, I think Teferi is just too good for such "cleverness," and I would save Ral for Izzet, Grixis, and Temur decks.
Not everyone plays Rest in Peace in Jeskai's sideboard, but I think Dredge is just so good and will continue to be quite popular, so I would definitely not skimp here (and in fact, I would play at least three dedicated graveyard-hate cards, to put alongside the Angels we should be sideboarding in). I don't think it's unreasonable to play a Surgical Extraction instead of the third Rest in Peace, particularly if you can squeeze at least one more Field of Ruin in here. The extra flexibility for other matchups is not trivial. Surgical the Urza's Tower is still a classic, and Surgical against Snapcaster/Cryptic decks can be just what the doctor ordered.
Holbrook doesn't run any, but I'm a fan of a couple Spell Quellers in the sideboard. The added counterspell (that can't be Duressed or Negated) is already nice against combo and control, and the body helps diversify our victory conditions after sideboard. Besides, many Modern combo decks are not safe to let live for long periods, as they can just kill you out of nowhere if you waste time, so the clock matters.
Lava Coil might not be flexible enough for this build of Jeskai - a deck with incredible sideboard cards - but it's probably not getting sideboarded as much as it should in the format, at large. Extra cheap removal is very useful in some matchups, and the ability to kill things usually out of Bolt range, as well as exiling dredge creatures is not to be underestimated. That said, Tarmogoyf, Siege Rhino, Tasigur, and Gurmag Angler all sort of suggest that, at least at the moment, four damage might not be enough more than three.
While a lot of Modern games are decided before there's time to drop a Niv-Mizzet, I suspect his day is not far off. In terms of a sideboard threat that just completely takes over the game, Niv-Mizzet is a 10.
While I'm warm to Niv-Mizzet, I don't see much future for Crackling Drake in Modern. While Niv-Mizzet would actually work great with a lot of what's going on in Modern (lots of great cheap instants, for instance), Crackling Drake is just a much worse two-for-one than we can get from a dozen other options.
While a respectable removal spell in Standard, Justice Strike is just too flimsy and slow for Modern. Too many opponents have no targets, and while Path to Exile can still be used on your own Snapcaster Mage to ramp, Justice Strike can't even kill every creature. If you've got a Justice Strike in hand, it's gonna get real awkward when your opponent drops a Tarmogoyf or Tasigur. Bolt and Helix can't kill everything either, but at least they can go to the face.
Here's what I'm looking at for this weekend:
Attach either one and the first time you draw a card, you loop as much damage as you like (since each damage draws you a card, which deals a damage). You know, the thing that was already true for Niv-Mizzet, Dracogenius and Niv-Mizzet, the Firemind.
That's kind of Niv-Mizzet's jam. He's just really curious.
Might need to call in Gabriel "Papa Hat" Nassif for this one...