Glad this has a spiritual successor! Actually was considering doing something similar to this, so why not make a big fat post out of it? Let's talk about some mons that jump out to me...
- Everyone knows how strong Solar Power Charizard is, now imagine that without the sun. Fire coverage alone is really good but most stuff that resists that is going to get nuked by Hurricane. Just for reference, full Sp. Def Toxapex has a 25% chance to get OHKOd from full by Specs Hurricane. The fact that it's decently fast and has a very good offensive typing would also help it force switches more easily. I'm not expecting this to be any good because of how consistent it'd be since it needs hazard removal to have any kind of defensive utility and relies on prediction but it should be fun either way.
- This thing gets Fluffy to help it set up easier. I don't think I need to elaborate much more because the implications of that should be obvious. Cloyster already has crazy physical bulk and is dangerous once it gets going so it might boil down to how well the meta's fat Water ends up answering this.
- Aerodactyl gets Comaphasing here. I don't think this was meant to be legal but I'd ban it if I were you. Kabutops also already knows Flip Turn so I'd mention that on the spreadsheet.
- I'm gonna be honest, I don't see this being immediate AG material. Keep in mind that this has a significantly worse initial offensive presence than something like Mega Mewtwo Y that wasn't even great in Ubers to begin with, or something like Ultra Necrozma that's very similar but has a base form that gives it a very strong defensive profile. It's not the fastest thing in the tier either which will mess with its ability to clean, and running Agility would either sacrifice some valuable coverage or further damage your initial offensive presence by losing Calm Mind. This is also Ubers where a Pokemon needs to be way more extreme to see a ban. Keep it for now, see if it's broken if you end up playing it.
- Magic Guard fixes the biggest issue this typing has, that being hazard weakness, while playing completely to Typhlosion's strengths as a Pokemon. This thing's going to be way more consistent in getting full-power Eruptions off and its Calm Mind sets are going to be significantly more dangerous than before. I love this Pokemon so I'm probably biased but this would be the Pokemon I'd be most excited to use if I were to play this meta.
- NatDex learnsets means that this thing gets Roost and Defog alongside Intimidate, meaning more defensive movesets are actually worth considering again. This thing's bulk is actually extremely similar to Landorus-T with the benefit of a typing that is both pretty good neutrally from a physically defensive standpoint and notably strong from a specially defensive one. I did notice Supreme Overlord and those sets might be alright too even if I don't see this thing being anything but a significantly worse Kingambit.
- Unburden could be really sick with this, you can basically run sets that would already be close to standard and just have doubled Speed. Like imagine Sub/CM/Ice Beam/Surf, does way worse against fat stuff but still might do fine enough and it anti-offenses extremely well now. This thing's natural bulk is actually super good, 100/115/115, so it's not going to struggle to get into Sitrus or maybe like Petaya Berry range. There's also the obvious terrain seed but at the time of writing this I have no idea if we'll get anything that sets terrain well or not.
- We already know how scary Greninja is with this ability, now cut some of the Speed and give it way better bulk and coverage and you have something potentially really dangerous on your hands. Potentially notable moves include Aura Sphere, Moonblast / Light of Ruin, Earth Power, and Shadow Ball. You also get Nasty Plot, Stealth Rock, U-Turn, and Synthesis as fun little options if you want those, and this also gets Seed Flare now for some reason.
- It's hard to say much about Hoenn since most of the more interesting mons in this part of the dex fall short imo but this is the closest we get to a big, relevant buff. You trade off the ability to hit Ghosts, something that admittedly is more relevant now than ever, for free 33% healing to make you resistant to chip, care less about stray hits, and whatever else. This thing's bulk is pretty nice for what it is even if its ability to come in on Ghosts is also way more limited. Thing mostly just does way better into fat teams now and I don't know what more you could ask for.
- Why was this thing unbanned exactly? Tough Claws lets its STAB moves hit almost the exact same power level as Adaptability did (1.95x vs 2x) on top of boosting Extreme Speed. Swords Dance sets were always what this Pokemon did best, so if anything this is a direct upgrade from the base form. I'd leave this banned if I were you.
- Quark Drive on this could be pretty cool. Don't know if I'd call it good exactly but this thing's really resilient to chip as is across Toxic immunity and Stealth Rock resistance so getting rid of Life Orb for Booster Energy could be nice too. You do lose Dry Skin though which was really what made this thing tick in the first place.
- Lumping these two together but god dang do they look cool. Magic Bounce and Regen really are awesome for these two, they were struggling in Ubers as is and this would breathe the life into them that they've needed for quite some time now.
- Probably the strongest thing here. This thing basically went from having no ability to having free unbreakable Sun to bolster what was already one of the single most dangerous moves in the meta. Still has literally everything else that made Victini so great in the first place, with the only possible exception being that it may have less defensive utility if Heatran and Tapu Lele and stuff go away and nothing pops up in their place. Looks really insane and possibly banworthy but I wouldn't quickban.
- Why is Chilling Neigh banned on this??? Heracross never used Moxie because of it having Guts which is straight up better and I see this not being any different of a case than that. Only real counterexample I could think is with Mach Punch off of a higher attacking stat but Fighting is still an imperfect offensive type with plenty of resistances so the counterplay of "faster Fighting resists" would still be there just like usual.
- This thing's always better in metas where it gets to not have Defeatist anymore but Purifying Salt is a really cool ability to go in its place here. It's not going to be completely game changing like something like Tough Claws or Magic Guard but it's a nice bonus for a mon like this to have. Its offensive presence is scary enough as is so this is how it should be honestly.
- Holy mother of jesus why did this thing get Speed Boost?? This thing was already really hard to switch into, it didn't need to be fast too... Its Speed tier isn't incredible at +1 but that didn't stop Blaziken from being Uber for 3 generations in a row, and unlike Blaziken this thing doesn't kill itself when it attacks. Probably locked into boots but Chandelure is going to be super dangerous now. So that makes two Fire/Ghosts that are way better now. Cool.
- Of course this thing would still get Limber. Very on-brand.
- I knew going into this that something fat would be getting Regen. Mandibuzz's typing has always felt like a bit of a double-edged sword for it, really cool resists but some awful weaknesses and a really big susceptibility to chip damage. Regenerator is going to help patch that up even if the problem will still be there for sure. This is the kind of Pokemon that'll fit on all kinds of teams now because of its good stats and Regen pivoting so I think this is a really great boost, mon's always been really close to good and I think this'll cement its place in OU.
- Trading a Ground immunity for a big, big boost to its Fire moves should be really interesting for this thing. I don't know how great Sun teams will be since I haven't been looking for abusers (although Pyroar has Solar Power which I just saw so maybe that'll be worthwhile) but it might not be bad for those as well, but in general having effective STAB Fire moves is always going to be appreciated.
- Cobalion's biggest issue was that it's always felt a little bit weak, and Guts is sure to improve that by a lot. This is one of few Pokemon quad resistant to Stealth Rock too which is nice, and it should still be fat enough to run Swords Dance. Coverage isn't perfect but it should be just enough for it to get by, and it's very fast for something this bulky. Cool mon, although it's kind of a shame it doesn't get Drain Punch.
- We already know how much of a menace Transistor makes Regieleki offensively, now here's something with less Speed in exchange for actual coverage. Legitimately could be super scary now, especially since this boosts its main pivoting move as well. Pretty straightforward.
- So the base form gets Contrary with Close Combat and will be fast enough to get at least one pretty consistently. It's still a fragile Pokemon and this won't fix all of its issues but it should be a fun little boost. It's worth note as well that this completely locks you out of Swords Dance and you also can't really run a choice item to make the most of this, but it's really not a huge deal. I doubt this'll have a place in the main tier, should be a fun gimmick option to mess with.
- Why was Kartana banned and Magearna not? Kartana's offensive typing is still horrible and the base power of its moves get in the way of what it could be capable of, and as nice as Shadow Shield is I don't think it'll break this thing. It'll be forced to run Boots to make use of it and it'll still struggle to break through the same defensive Pokemon that have always given it trouble. Magearna was not broken by its ability - it was broken by the fact that Choice Specs and CMSG were both very overtuned sets on their own and had entirely different counterplay, so it was very easy to, say, send your Toxapex out expecting CMSG only for it to blast you with Thunderbolt or Trick you its choice item. Now it gets Spikes as well because it needed that.
- Okay, one uninteresting Gen 8 later and we have more ban talk. Neuroforce should not be banned from Urshifu. This thing broke stuff by hitting it neutrally, not super effectively, and winning matchups that you already win even harder will not be a problem for a Pokemon like this. The only real impact I see it having is like chipping something U-Turn weak down more on your Choice sets which is hardly going to break it, especially with Swords Dance being a bigger issue. And like Magearna, Grim Neigh was not the problem with Spectrier, it's that this thing could run basically whatever set it wanted and was absurdly strong, fast, and just bulky enough for its typing. Mandibuzz definitely being better would not fix how centralizing this thing will inevitably be. I'm starting to find more and more gripes about your bans the more I go down (Soul-Heart Cyclizar is banned????) but it's best to not flood the post with this and just stop here.
- Back to other cool Pokemon, Spidops gets Toxic Debris now. This perfectly fits the kind of job this Pokemon is trying to have and the base version is bad enough to the point where I think this is a perfectly fine boost. Might even see usage on teams that don't really care about Webs that much, which is honestly pretty cool. I don't think Webs is very good and pinning it to an a Pokemon your team might want anyways is an interesting thing here.
- Tinted Lens! It was already difficult enough to switch into this thing and it's going to be a nightmare now. The only remotely relevant Pokemon that quad resists Dark is Grimmsnarl unless there's something weird I missed that suddenly becomes relevant.
- Lastly, this thing ended up with Iron Fist somehow. It's going to heal a lot more off of Drain Punch and in general a lot of the moves it already enjoyed running are going to be a lot scarier now. Iron Hands has felt like it's just barely on the edge of viability in Singles and this little push could go a really long way.
That's all I have to say for now. This isn't all that I could talk about, like I think Boufallant, Dodrio, and Zeraora could potentially be cool too. I was running out of steam as I got later into the post though so I figured I'd end it off there.