Here is a Roster I made. Somewhat plausible, somewhat wishful thinking. 74 characters.
Newcomers:
Captain Toad: Part of me thinks he is shoe in. He appeared in every main 3d mario game since Galaxy, he has his own game, if that is not enough I don't know what is.
Kamek: A quite big Mario character, specially in the Yoshi branch, with a lot of moveset potential.
King K. Rool: If not him, Dixie. But it's past the time for a DK newcomer. I prefer the villain.
Pauline: Original maid in distress, relevant once again after recieving acclaim for her appearence in Odyssey. Can mix both retro and new elements frm the Mario/ DK series.
Urbosa and Mipha: Or any BoTW Champion. They have lots of moveset potential. Urbosa can mix an arabic fight style with electric magic and Mipha is a cute water dweller with a trident. They also represent two very recurent races within the Zelda universe.
Rex and Pyra: Uniqueness and relevancy is on their side. The driver and blade mechanic could make an awesome fighter.
Marx and Medusa: Sakurai's babies. The lowest of the Kirby villains that has recency and potential and a female villain that is not Wendy'O Koopa.
Ridley: Wishful thinking.
Decidueye: Badass looking grass type starter that represents the most recent Pokémon games.
Inklings: Confirmed.
Ribbon Girl: Arms needs marketing.
Labobot: The WTF character that represents the switch era.
Takamaru: The retro rep.
Shovel Knight: He looks and feels like nintendo star. He has got even an amiibo. He is a Knight with a freaking shovel as a weapon.