Fair warning, these are heavily influenced by postcov/ Vic Chaos bc I love him:
-Quite tall, not necessarily fat but he was a chubby kid and continued to be until young adult years. No one gets a nickname like "Butters" if they're skinny.
-Other than being naturally good with numbers, he's not particularly talented at anything. He manages to get solid grades at school (like high Bs to low As) simply because his parents pressure him to work hard.
-One of those asshole rich people as an adult who cares way too much about flashy cars and watches. For vacations he goes on cruises during the summer to go to places like the Bahamas or Caribbean. Probably an asshole BMW driver at one point or another.
-Preferred method of tax evasion is the annual gift tax exclusion. He's very... charitable. Hm. Yes.
-This isn't so much a headcanon, since there's canon basis to it, but he enjoys expressing himself creatively, mostly through dancing and arts & crafts. Idk, it's funny to me to imagine a shady businessman with no moral backbone also take up pottery classes in his free time.
-He very strongly prefers to top. He's always leaned towards having a conservative view of gender roles, and I think that manifests itself in the bedroom where he wants to be the 'man' and fulfil whatever expectations he thinks that comes with (barring Marjorine, but honestly I can work w/ that). Also enjoys having power for that reason, though I can also see it as being a way for him to take control in some parts of his life after being pushed around so much as a kid. The only times I can see him bottom is with someone he really trusts, or when he's inexperienced and hasn't been able to do things the way he wants yet. Also, he's hung.

Some additional bunny-centric AU thoughts for funsies, because I have some fic ideas that are kind of relevant, so they've been on my mind:
TFBW: As Prof. Chaos, he has a crush on Kenny but hates Mysterion, and kind of gets stuck in a weird love triangle/love square situation because of it. There's a moment where he finally captures Mysterion, is super psyched about getting to reveal the identity of his arch nemesis, then pulls off Mysterion's hood and his brain short-circuits. Cue mini emotional crisis that gets resolved with a newly developed bondage/rigger kink.
TSOT: I like the idea of Butters training alongside Stan, maybe with Randy (who holds the title of some form of nobility, but I don't know enough about that atm to say anything specific), who sorta takes him in as an apprentice bc his son could benefit from having another boy his age to train with. Anyway, I think he gets a lot of shit for not being good at first, only to then outperform Stan, much to his horror. Perhaps Butters becomes particularly motivated after falling for a certain Princess he must vow to protect :3c

-to the surprise of absolutely no one, i'm a puppy stan truther
-gets put on SSRIs as a teen. they make him gain weight and his dick barely works but he's a bottom so whatever. he knows he shouldn't drink alcohol while taking them but sometimes does anyway so he's probably really fatigued all the time and has high blood pressure
-besides being overweight he's average in everything else
-vegan in principle, not so much in practice. he tries his best, mostly for the animals but also to balance out the other ways he's fucked up his health (see above), but i think he'd struggle to turn down a bacon cheeseburger after a particularly bad hangover
-still into metal music even after the crimson dawn era. he attends local shows and his wardrobe consists mostly of obscure band tees with indecipherable logos, but it's good no one knows what they are because they're all called, like, Necrosadist or Carnivorous Vagina (real bands, btw).
-in a similar vein, his fashion taste is awful. it's non-existent. his idea of dressing nice is putting on jeans instead of sweatpants
-after he moves out, he gets distant with his parents (cause fuck Randy) but his relationship with his sister improves. Shelley treats him better and they reconcile
-she can be transfemme if she wants! like typically i still think stan is cis, and i don't get super passionate about any particular gender or sexuality headcanons, but if i have to choose anyone out of the m4 to be a trans girl it'd be stan. she's had a phase where she DIY'd hormones too

sorry, i know a few of these are already super popular hcs, i don't think about him too often but this was a fun thought exercise regardless! thank u anon <3

This really is the perfect first ask to get, but I'm going to ruin it by listing my top 3, because I can't choose just one.

Mysterion Rises: I love superhero stuff. I love Kenny. Doesn't get much better than that. The superhero trilogy in general was great, but there's still a bunch of loose ends and things that were left unanswered, and I actually have a big AU brewing in my head that ties together a bunch of Lovecraftian entities into Kenny lore... that's a topic for another time though.
Christian Rock Hard: Tolkien and Cartman's interactions are so funny, and Butters farting on Cartman and flipping him off at the end is just chefs kiss.
AWESOM-O: Butters being an adorable naive idiot for an entire episode is just classic South Park, but he does get a win at the end, which bumps it above the rest of Butters being adorable and naive.