the big one for me is any Undertale or Deltarune character whatsoever. if i had my pick i'd say either the Fun Gang or Undyne, but i'm serious about taking literally anybody; watch me main Burgerpants, i don't care. they'd all be loads of fun and visually distinct, and half the appeal is that sweet, sweet Toby Fox soundtrack anyway
Cadence from Crypt of the Necrodancer could be fun too. I'd be curious to see how they translate a rhythm-based game to Smash. and like i said with Undertale, the music would be amazing. (hmm, would Cadence of Hyrule music play on her own stage, or the Zelda ones?)
i still haven't started up Shovel Knight, Hollow Knight, or Shantae, but they all look like fun possibilities. i'm too weak to play Cuphead but the whole aesthetic is super charming, i'd welcome him
and one more, but not as a playable character - i've always thought Baba is You would make for a bonkers assist trophy. Baba or Keke or whoever would show up in the background and make or break rules to change how the battle operates; break [character] is You to disable the other player's controls momentarily, turn all the scattered weapons and tools into apples with Item is Fruit, or set up Floor is Lava to make the obvious happen