Why Belmont?
There's nowhere near enough of a connection, in my opinion.
Bomberman would be a better call, but I think he'd be too limited moveset-wise.
Due to the role of villains this time around, I'm thinking it's possible that last spot could be one, but I really can't think of anyone suitable, looking at the roster. K. Rool seems ruled out through the explanation of DK's involvement being Bowser, the Mario universe already has 3(+?) characters, as does Kirby's now. With Ganondorf returning, so would Link/Zelda's, and the only villain suitable I can think of to add to Fox's universe I see being a Fox clone (Wolf).
The third-parties seem stand-alone, if you ask me, since they are essentially guests, and Pit's role as an onlooker in the SSE intro makes me doubt that his nemesis(es?) will feature, though I know -nothing- about Kid Icarus. As far as I'm aware, the Ice Climbers don't have a counterpart, so if this newcomer is going to be related to anybody, it's going to be Lucas, Ike or Samus as far as I can tell.
Did Samus have a single, set enemy? For such a popular series, Metroid seems under-represented...