The fanbase would be the problem you say? We went from "3rd party characters are taking all the slots" to "3rd party newcomers are more hype" pretty quickly. I'm with Gengar on this one. And I don't even want it to happen.
Maybe they'll just add very few non videogame characters first like they did with Brawl, maybe it'll start with just Goku, cause honestly I think Goku easily has the most fan support of any character imaginable. And again, I don't like it either.
And yeah I know the Disney stuff is very limited, but it's still there. That's them being half way on-board in my eyes.
I personally want first party owned characters most, Dixie, Cranky and Funky Kong, Isaac (with a Felix Echo plz), Impa, any of the Champions of Breath of the Wild (Urbosa or Daruk are my preferences), Dimitri from FE3H, any sort of beast Ganon, Skull Kid, Chozo Warrior who must not be named from Metroid Dread, Toad or if they must Captain Toad... Those are characters I want the most. A favorite of the 3rd party possibilities is Monster Hunter, but he's a huge exception.
Sponge Bob, Mickey Mouse, Goku, the Hulk, Batman and the Ninja Turtles can stay out.