And to pre-empt the question, yes Geno is owned by Square Enix, thus his inclusion would mean a Third Party Character
In Sakurai's own words his status is unknown and they don't know the legality of using him again. As well he is as much a "third party" as Sora is for Squarenix. It is why some people describe characters like Geno and Sora as 2.5 Party characters (or 2nd party characters)
I certainly do want Geno, I just don't think he has a chance.
Brawl was pretty much the last chance.
Usually it's fair to treat such middle ground as Developer Ownership. It's the same reason about half of the Characters in DK64 are considered Rare's property.
Anyway, thought I should out my hopes for character changes
Removal of Lucas - While unlikely with Sakurai at the helm, Lucas really is a clone without a purpose, whose slot would work better with someone else (though I do like Lucas as a character).
Removal of Wolf, De-Cloning Falco - Star Fox really doesn't need 3 slots for variations of 1 character, and it would be nice if Falco became more unique
Further Decloning of Luigi and Ganondorf - Both deserve own movesets, not much else to say.
Ike -> Chrom - While difficult to incorporate Chrom's moveset to not be a clone of Marth's, it fits the natural progression that Pokémon may have with the Mewtwo slot.
Beyond that, I really don't have an opinion on who should be added to the roster.