I agree with what seems to be the general consensus here, which is that
Heatran isn't quite S rank material. It still suffers from its biggest problem which is the ease at which it gets worn down. The huge popularity of Spikes on both Protean and Ash Greninja really exacerbates the problem; Heatran really, really hates Spikes, so that's an extra thing it has to deal with. Defensive sets are pretty passive and invite in stuff like Keldeo, while offensive sets have real trouble switching into stuff like Specs Tapu Lele. Not to mention, the metagame is so prepared for it that it can't reliably switch into the things it's supposed to beat: Magearna often runs Focus Blast, Volcarona runs HP Ground, Venusaur is starting to use EQ, heck even Ferro can cause problems for it with Knock Off. I don't see it as that much better than the rest of A+, and I cerntaly don't see it on par with Magearna or Landorus-T which, to me, are S rank in a large part because of how offensively threatening they an be.
Volcarona needs to drop, I think that's pretty obvious. Even before Arena Trap got banned it was starting to have real problems, and it's gotten even worse since. The fact that it barely sees any usage at all on high ladder speaks for itself to be honest. I'd argue it could actually drop to A- rank, though teams being less prepared for it is definitely an asset since it can run over them. But it still needs quite a bit of support and it's not something you can just put on a team and expect it to work anymore.
Hoopa-U definitely needs to move up to B+ if not A-, it's a formidable wallbreaker that can put in work vs offense thanks to its special bulk, taking advantage of stuff like Mew and Zapdos. Magearna causes problems for it, but it can only switch in so many times especially if it's offensive (which most are), and sets that aren't offensive aren't really that hard to deal with. The fact that it's one of the few physical attackers that can break past Toxapex makes it really potent vs stall, until they all start running Banded Weavile again at least.
I also agree with a
Mimikyu rise, its ability makes it a fantastic asset to HO as well as a really deadly sweeper. If you can capitalize on the mindgames around it, it can be absolutely brutal since it's actually pretty easy to get it all the way up to +4. IMO It's a much easier fit on any team than anything else in B rank, you can literally just slap it on any team that needs an emergency check that doubles up as a cleaner. It has a strong and valuable niche and is defenitely at least on par with Weavile and Gyara.
OK, I have a nom of my own, and that is
Ash-Greninja to S rank. It might be a long shot and i'm not entirely sure about it myself tbh. But I do think it at least merits discussion.
Right now, Ash-Greninja is easily one of the most formidable threats in the tier, as it has been for a while. It absolutely destroys teams that lack Dark resists, and its most common checks such as Ferrothorn, Keldeo and Tapu Bulu can't switch into it repeatedly and don't need to be worn much for it to 2HKO them, at which point it's pretty much unstoppable. On top of that, Water Shuriken gives it minor revenge killing capabilities and make it much harder to stop once it's transformed.
What makes it S-rank worthy to me are Spikes. Ash-Greninja is an absolutely fantastic Spikes setter, not only because of the amount of switches it can cause, but also because the tier's man defogger, Mew, absolutely does not want to stay in against it. Tapu Fini is still a pretty big problem for it, but it's not too hard to take advantage of either, and its usage has been on the decline recently. The amount of pressure Ash-Greninja can put on a team is considerable, especially since Spikes wear down a lot of its best checks. Spikes in general are very potent and Greninja is easily its best setter, and personally, I find Ash-Greninja to be a better Spiker than its Protean counterpart since it doesn't suffer from the same 4MSS.
It's an absolutely meta-defining threat overall and it feels a step above everything else in A+ rank, I'm just not sure if it's on the level of Magearna and Landorus-T which, on top of being considerable threats, provide a huge amount defensively which makes them more splashable. However, I have found Ash-Greninja very easy to fit on teams as well, almost any playstyle but stall can make very good use of it.