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