Considering the fact that this game has already been released (and sold over a million copies) in Japan, chances are Nintendo isn't taking any more character suggesstions...
For the NEXT Smash Bros. game, however...
Megaman (Classic or X; please no "Battle Network" version!)
Tails
Knuckles
Shadow (come on, Shadow is the coolest new Sonic character since Knuckles)
Revolver Ocelot (from MGS)
cybernetic Raiden (from MGS4)
Dante (from Devil May Cry)
Vergil or Nero (from Devil May Cry)
Tingle?
NO Shadow, period!
I'd like to see in the next one (if there's going to be a fourth Smash Bros.)
Baby Mario and Baby Luigi
Bring Mewtwo back
Alternate Doctor lab coat for Mario (They let us choose between Wario's Biker outfit and his original attire, why couldn't Mario have his doctor lab coat as an alt costume, and sort of bring the Doc back?)
Knuckles for Assist Trophy
Megaman or Bomberman (but not both)
Samurai Goroh for playable character
More Metroid characters
Fawful for Assist Trophy
Sonic needs to be voiced by either Jaleel White or Ryan Drummond