For characters that I want to see, Dante, Doomguy, and Geno would be at the top of that list, in no particular order. Rayman or Elma would be great, though I feel like the odds for them might have been better in the base game, and I'd like to see what they could do with Agumon, Travis Touchdown, Amaterasu, or Edelgard (as long as she's not another character with a counter special - I'd like to think that there's enough material in Fire Emblem for them to come up with something original). Honestly, I could go on all day about characters that I'd be interested in; after we got Snake and Bayonetta back, Ridley and K. Rool in the base game, and Banjo and Kazooie in the DLC, I'm not exactly picky.
For characters that I don't want? ...Jonesy from Fortnite? I can't say that I'd be thrilled about Monster Hunter getting in (my only experience with the series was Tri on the Wii, but the gameplay seemed really clunky, and there are at least half a dozen Capcom characters that I'd prefer off the top of my head), but that's all I can think of in terms of semi-realistic picks.