This gen seems primed to make a big impact on OU and I'll be excited to test - I know people always say that, but seriously.
Rillaboom: Grassy Surge is no joke, and it boasts more HP and speed than Tapu Bulu (at the expense of worse...everything else and no fairy typing). Rilla has several of the things that make Bulu great (Wood Hammer, Swords Dance, Bulk-Up, Protect, Substitute, Superpower) and several great utility side-grades to make up for not having Synthesis and Horn Leech in the form of having access to Knock-Off, U-turn, Endeavor, and its signature Drum Beating. Oh, and it has access to High Horsepower, Darkest Lariat, Acrobatics, Drain Punch, and Body Press for even more coverage options. This monkey means business.
Cinderace: I actually think it might be a bit overhyped, as it is slower than Greninja, has no priority moves, and in general has a muuuuch worse movepool for abusing Protean Libero. Still it has some good stuff - Gunk Shot is wicked stuff coming off of a base 116 attacking stat and it makes Cinderace the most powerful "Poison" type in the history of the game (minus Arceus-Poison of course). Cinderance might also be able to utilize Protean defensively in a way Gren couldn't with slightly better bulk and access to a decent steel move in Iron Head - getting to become a Steel-type on the fly is pretty powerful. I think "Worse Greninja" might end up in UU...if it wasn't for his seemingly exclusive access to the hot new move Court Change on top of all this. We'll need to test to see if Court Change is lolbroken or "just okay" but on its face the ability to completely flip the script on hazards and set-up is a HUGE deal.
Obstagoon: I was ready to write off this clown based on his Hoenn-versions' uselessness and the fact that Dark/Normal is a horrendously poor defensive typing. But then I saw its exclusive move and 520 BST. In the CAP forum it is often discussed that King's Shield is just unusably broken - the move turns matches into 50-50s and even on a 'mon without Stance Change it is busted. Obviously Obstagoon's Obstruct isn't as powerful - Screech is a lot less powerful than Feather Dance, tactically speaking. But, Obstagoon has more or less the perfect stat spread to take advantage of this with balanced and reasonable physical sweepeiness backed by solid bulk. His STAB options sorta suck and that's most likely the thing that kills him (Return, Double-Edge, Night Slash, Throat Chop :( ) but for coverage he's got Cross Chop, Close Combat, Gunk Shot, Iron Tail, Stomping Tantrum, and the elemental punches which is all pretty good. Flame Orb + Guts might just be what he needs to wreak some havoc.
Polteageist and Cursola: They learn Strength Sap. This is not a drill - these two ghosts have +100 BST over Shiinotic, have actual good abilities that further steal momentum from physical attackers, have better movesets and stat spreads than Drifblim, and they learn Strength Sap. I don't know how good either of them are (Drifblim is PU even with Tailwind and Defog) but Strength Sap is a move that causes frequent panic attacks in the CAP moveset discussion, so it is notable that we got not one but two new users.
Toxtricity looks like a CAP-mon. We have a clear niche with a powerful special attacker - Boomburst off a base 114 SpA is pretty sick, and that's before we factor in whatever bonus Punk Rock grants (god forbid it's a full 50% for "STAB" Boomburst). Even if Punk Rock is just 30% that'll still make its signature Overdrive hit a smidgen harder than Thunderbolt with perfect accuracy compared to Thunder. Sludge Wave rounds it out with surprisingly solid coverage or there's utility in the form of Volt Switch, Taunt, Acid Spray, Thunder Wave, and Shift Gear. I suspect that's the better set - but speaking of Shift Gear, unlike Magearna it boasts a reasonable list of physical attacks including decent STAB with Wild Charge and Gunk Shot and an amazing coverage/utility move in Drain Punch. It has the option to Technician Nuzzle or Acid Spray, too, but the first two options are probably just way better and combine to make an exciting, unpredictable threat.