Namco-Bandai could make a truly ultimate console mash up game with so many of their franchises that could rival Smash Bros if they truly wanted. I know there is that one game they are working on, but they really should go for something big with it.
Gundam
DBZ
Naruto
One Piece
Tales of
Digimon
Klonoa
Soul Calibur
Tekken
All the other anime/manga franchises they have access too.
I think something like that wouldn't be plausible, mostly because dealing with lots of different IP owners makes it increasingly harder to release anything (like Jump Superstars, which was only released in Japan because IPs were spread accross too many companies in other regions). I'm not sure who the IP holders for those franchises are in the US, though, so it might be easier to make than I think.
I'd really like it if they gave Ganondorf's current moveset to Black Shadow and came up with something more in-character for him but, sadly, it's unlikely that they'd add a second F-Zero representative when there're other, more important characters people want... unless they're planning to bring the franchise back, that is.
They might switch Lucario with a new, current-gen pokémon. Zoroark could be cool, but Pokémon X/Y will probably be out by then, so they might end up choosing a generation 6 'mon. I'd love it if they'd add Mewtwo back this time, but I guess that won't happen since 75% of pokémon characters come from generation 1.
As far as additions go, I'd love to see a castlevania character in there. Most Belmonts would probably play a bit too much like Link, though, so they could always go with a different (albeit less iconic) character such as Soma or Shanoa.
I might be okay with three or four new characters and some fixes/balances to the previous ones, such as Samus not being nerfed so much or Ness getting better range for his recovery move (why did Lucas gets longer PK thunder range, better aerial maneuverability
and the rope snake?). Oh, and don't forget nerfing Meta Knight without going overboard with the changes like they did with Kirby, Pikachu and Ness from SSB64 to Melee.