mass theorymon time :
Genesect: It's been a long time since I've used this thing, but I remember it being an obscene momentum machine which provided amazing support to offensive mons by checking a variety of threats and being an excellent offensive pivot. This thing could theoretically be good for the metagame in a centralising GSC Snorlax kind of way since it checks huge threats like MAltaria and Fairies in general, as well as giving us a very good scarfer which OU lacks (Lando-T is the closest there is to a good scarfer atm and I personally find it mediocre).
However, its incredible versatility and godly coverage can easily put this thing over the edge, its only real counter is Hetaran who is easily worn down, literally everything else gets screwed by coverage bar Rotom-H which is even easier to wear down. Most people expect it to be choiced so play around it accordingly only to get bodied by its coverage (I used to run Assault Vest back in the day which caught a lot of people off-guard)
It's a very mindless and centralising Pokemon, and once the joy of trying this thing out again dies down, people are probably going to get sick of its presence very quickly.
Mega Mawile: I guess the popularity of Tankchomp, Hippowdon and Bulky Talonflame are annoying for it, but it's still a completely ridiculous threat with 2 amazing sets, SD is stupidly difficult to switch into, just to give you an idea it's likely to OHKO standard Hippowdon at +2 which is just lol, and Subpunch can really screw over WoW users like Talonflame and Heatran. It also checks Pokemon that have gained in popularity since it got banned, namely Clefable, Weavile, Torn-T, Alakazam, so in many ways the metagame is even kinder to it now than ever.
I remember this thing being way better in practice than in theory, mostly because the amount of switchin opportunities it got was way too high compared to the things that could actually switch into it in return, and also because almost all counters to SD just lost to SubPunch and vice-versa.
So yeah prepare to see sets like WoW Heatran and WoW+Flare Blitz Phys Def Talonflame because Mawile is going to be brutal and will require extra preparation.
Blaziken: This thing is certainly more manageable than back when it was banned. In early XY, HO was really the only playstyle used, and it was a playstyle Blaziken absolutely demolished, the only answers to it on there being Talonflame and Azumarill. But now, people are using more balanced teams which, to paraphrase the OU council, did not work in that metagame, with stuff like Hippowdon, Slowbro, Altaria and bulkier Talonflame variants (back then all everyone used was Band which died super fast) being more popular.
But still, this thing is a truly deadly and and mindless sweeper. Because it can run an Adamant nature and has high BP STABs, it hits really hard after a boost : for instance, at +2 it has a good chance to OHKO standard Hippowdon with HJK. While its bulk isn't stellar, it can still find a handful of opportunities to set up SDs thanks to the large number of switches it forces vs bulky Grass- and Steel- types, and has the typing to set up on popular Pokemon like Bisharp, Scizor, and Weavile. I think this thing will be pretty devastating and will make a handful of Pokemon nigh-unusable.
Greninja: Has only gotten better since early ORAS, the only common answers to the standard Gunk Shot / Low Kick / Ice Beam / Dark Pulse are like, Klefki Chansey and SpD Sableye and that's it. Singlehandedly and effortlessly demolishes standard balance cores, has the speed tier to outpace most major threats besides the 4 speedster megas, Talon and Weavile, the latter of which loses 1v1 even if it carries Low Kick since that does not OHKO from full.
Not much to say about Greninja besides the fact that it is, by all accounts, completely and utterly broken, even more so than when it was suspected.
Reshiram: This is probably the most broken of the lot honestly, and I'm not talking about Specs or Scarf, I'm talking about a bulky set of Blue Flare / Draco Meteor / WoW / Roost, which not only has the firepower to still be an effective attacker, but completely insane bulk combined with Will-O-Wisp and an amazing typing, acting as a WoW spreader as well as a general answer to Electric- and Steel-types. It's basically bulky XZard except a lot better since it doesn't take up a Mega slot, has much higher special bulk, can run Leftovers, has access to 2 130 BP STABs with no drawbacks besides a SpA drop from Draco Meteor, and can run its dual STABs without being stopped cold by Heatran thanks to Turboblaze (in fact, its STABs are only resisted by Azumarill and Diancie, the former of which is WoW fodder, and the latter of which usually runs a -SpD nature and thus is 2HKOed by Blue Flare after SR with little SpA investement).
I mean, even without even any bulk investment it actually walls LO Thundurus lol. This means it can run something like 248 HP / 68 SpA / 188 Speed with a Modest nature, and still hit like a goddamn truck (can 2HKO standard SpD Heatran after SR with Blue Flare just to give you an idea) while speed creeping Bisharp and being super bulky.
Yeah it's weak to SR, but honestly this thing is so good it can mostly do its job even when SR are up. Reshiram probably won't even be close to balanced given its combination of bulk, power, and coverage.
Deoxys-D and Deoxys-S: Deo-D will probably be slightly easier to handle than it used to be because of the presence of good Magic Bouncers and how common Starmie is. However, it's still a ridiculous boon for offensive teams whose ability to guarantee SR and at least one layer of spikes can force tons of pressure on most teams when given support from something like Weavile or Bisharp. It can also act as a very reliable, very bulky hazard stacker for more defensive teams.
Deo-S on the other hand kinda suffers from the ubiquity of Scizor, SpD Skarm, Clefable, and Fairies in general, however it's still a completely insane revenge killer, one that isn't limited by Scarf and has access to very good coverage and mixed offensive capabilities.
Don't think these guys will be too much to handle per se, but they won't be fun to play against, and their presence will probably make for a staler and less enjoable metagame.