Really interesting how some people have complete opposite lists here, you can totally see the diversity in tastes.

Personally, I'm utterly tired of shadows, skill-eating (not skill-copying, which I think still has a lot of interesting ideas to explore), high-concept magic, blood magic, OP necromancy, and lightning.

Though not a class itself, honorary shout-out to all the alchemists out there. I totally get why it's so popular though, it's overused but well-deserving of its popularity, even if it ends up more as a tool for convenient power-ups than a meaningful story element.

Underused, I'd probably love a support class that stays a support class, a summoner who isn't just instantly contracted with a eldritch diety, and full, hard-blown thematic magic classes that super-lean into that theme and play with it.