Here's some (non-exhaustive) thoughts about what I expect to change about the sets mons currently run in OU.
Terrakion can be soft-checked by our new extremely physically defensive additions Slowbro and Tangrowth to varying extents. Additionally, Azumarill can check it offensively as well. Personally, I expect it to fall off a little bit. The primary change I expect from Terrakion is the incorporation of
Megahorn into its banded sets, resulting in a set of Stone Edge / Close Combat / Earthquake / Megahorn. The uses of this move were pretty fringe, but Slowbro's and Tangrowth's addition push it into relevance. Megahorn 2HKOs physically defensive variants of Slowbro and Tangrowth easily, while CC is a non-guaranteed 2HKO on phys def Tangrowth, and Stone Edge doesn't even 2HKO phys def Slowbro. It also hits some weirdos like Celebi and Mew.
252 Atk Choice Band Terrakion Megahorn vs. 248 HP / 252+ Def Slowbro: 264-312 (67.1 - 79.3%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Choice Band Terrakion Megahorn vs. 252 HP / 252+ Def Tangrowth: 240-284 (59.4 - 70.2%) -- guaranteed 2HKO
Pretty good stuff. I think it's unlikely that we'll see Megahorn on anything besides Banded (or scarfed if that comes into favour) sets but it's a pretty cool new tech that Terrakion got this generation that it can put into effect.
Cinderace doesn't actually handle Slowbro very well at all. The typical Bulk Up set usually only has Sucker Punch as the best move to hit it which just is way too inconsistent. I personally expect to see more
Gunk Shot on Cinderace instead. Gunk Shot at +1 has a small chance at 2HKOing physically defensive Slowbro, improved if you factor in Gunk Shot's poison chance. Gunk Shot also handles Primarina, Azumarill, and can do a bit more than average to the ostensibly fringe Talonflame which some people brought up as a Cinderace check. If you're picking coverage to complement this, you'll probably want Zen Headbutt for Toxapex and I guess Pyro Ball for Aegislash + other steels? Resulting in what I expect to be the prevailing BU Cinderace set, which is Bulk Up / Gunk Shot / Zen Headbutt / Pyro Ball, with potentially Sucker Punch replacing one of the moves. This coverage set only really misses out on Tyranitar
and Heatran. Unfortunately, it makes it so you need to time your moves really cleanly against Hippowdon. It should do just fine, but there's also the possibility that Slowbro's presence will just make BU Cinderace completely not worth using at all, which should be kept in mind.
+1 252 Atk Libero Cinderace Gunk Shot vs. 252 HP / 252+ Def Slowbro: 183-216 (46.4 - 54.8%) -- 9.8% chance to 2HKO after Leftovers recovery
Kinda depressing though isn't it? Even at +1 it still does relatively little to Slowbro.
But what if I told you there was a secret tech that obliterates Slowbro? You're surely thinking Super Fang, which I think could be a nice way of keeping both Slowbro and Toxapex low. But I'm thinking of
Electro Ball.
0- SpA Libero Cinderace Electro Ball (120 BP) vs. 252 HP / 4 SpD Slowbro: 198-234 (50.2 - 59.3%) -- 76.2% chance to 2HKO after Leftovers recovery
Previously this shitmove was only used to meme on Toxapex with about equal-or-worse effectiveness as Zen Headbutt, but Slowbro actually gives this move a valid niche! With a little investment, you can even 2HKO phys def Toxapex after rocks with this, giving it potential as an option to cover both Slowbro and Toxapex in one slot. Sorta. Definitely something worth considering for non-Bulk Up sets but will likely be a meme on the level of Timid Kartana.
I think Chansey's presence and the ostensibly increased importance of stall as an archetype will actually serve to kill a lot of Dragapult's viability (sad but true). First of all, Specs will take a huge drop since it really can't do anything to Chansey except just U-turn out, which is fair I guess. Wisp + Hex variants similarly accomplish very little. Even if they run Substitute to wall Chansey, Dragapult still can't really do all that much and it becomes just a PP stallfest. If Chansey has Teleport it can pivot out to heal its burn, bring in something else safely, or just stall like that through pivoting back and forth. Bear in mind that Teleport has a healthy 32 PP that can be used to burn through all of Dragapult's PP pretty easily. And you'd think that Dragon Dance variants have little trouble with Chansey but sadly that is just not the case. The ladder Sub + DD sets have a big issue now, which is that most bulkier teams will be running a Normal and a Fairy type, meaning that due to Phantom Force's charge turn, the stall player can just pivot back and forth between Chansey and Clefable until they run out of PP if they're running Phantom Force + Dragon Darts. This has always been kind of a problem for this set, it's just that Normal types were much less common until now. So it kinda necessitates Steel Wing for Sub + DD sets, which runs into issues with coverage and power since it's just such a weak move. DD + 3 attacks fares better, but Phantom Force is still quite risky when Normal types are much more common. There's also the chance that if hard stall rises as a playstyle, we could see Unaware mons rise to greater relevance, all of which are more than capable of handling a DD Dragapult. So, based on what we know so far, it doesn't look too great for Dragapult. Tutor moves + unknown additions could result in this shaking out completely differently, but we'll see.
Now these other ghosts should do just fine. They can sub up on Chansey and break through it with CM (Chandelure) or NP (Gengar). Gengar and Aegislash being immune to Toxic means that they can just set up in Chansey's face usually, even without Sub. Gengar and Chandelure can also trick their choice items onto Chansey. But I still expect Choiced sets to fall out of favour a bit on these mons (especially Specs Aegislash) as even though Chansey can't touch them, it is still capable of gaining momentum against them thanks to its new toy Teleport. Chansey can easily pivot in, heal up if needed, then Teleport out to something that can threaten these Ghost-type breakers back, like Ditto for example.
Similarly, Specs can't break Chansey (or SpDef Scizor), so we'll likely not see it as much. SubRoost seems nice but honestly loses a lot of its luster now that we know how Metronome works. I expect that SubRoost will become the standard, and we might even see SubDD as well, but Specs won't die out completely, it's still a very effective balance breaker.
Chansey really shits on these guys' NP sets. I don't really think most of their sets would change, just that their effectiveness might be cut a little bit. I still expect Modest NP LO Hydreigon to be a thing, as Modest guarantees an OHKO on Chansey with +6 Draco Meteor. NP Rotom just can't do a lot to Chansey at all unfortunately.
Things are looking up for Rillaboom imo. Magnezone's presence to trap Steel-types, ostensibly less Dragapult, and the fact that certain teams will be dropping Toxapex for Slowbro as their bulky water of choice are all things it likes, not to mention its new move Grassy Glide. Amoonguss checks it sure, but Tangrowth actually doesn't check it as well as you might think unless it runs Sludge Bomb... which basically just hits Rillaboom and nothing else. I mean I could see it running it but the moveslot competes with other things. Someone posted about a Grassy Seed + Acrobatics set which honestly looks legit for killing Tangrowth and Amoonguss (as well as Kommo-o) too.