Sorry if I'm repetitive, as I'm sure I will mention things that have already been said, but these are some characters I'd really like to see.
Geno- I mean really, he was such a BAMF in Mario RPG, and he made an appearance in M&L:SS. Why not? On a similar note, Mallow would make a good Assist Trophy...
Improved Mewtwo- Let's face it, he was pretty lousy in Melee... but a few moveset tweaks and a tad bit more weight would do him justice.
Knuckles- I was saddened by his absence in Brawl... I mean /really,/ if Sonic's in, why not Knuckles? He could have a very interesting moveset, and the whole gliding thing incorporated in Brawl would have been great.
Deoxys- I know his chances of making it in a future SSB game are pretty much nonexistant, but hey, this whole post is a stretch. A simple forme switch mechanic could be implemented, where his weight and other similar properties were changed depending on whichever forme he was in.
Giegue- I personally find the character of Giegue, or Giygas to be quite... interesting, and rather scary coming from Nintendo. I think he'd be a neat addition to the next SSB, and his Final Smash could be Giygas' unstable form...
Will add more later, gotta love getting kicked off the compy in the middle of a post.