New characters:
Chrom (Fire Emblem Awakening) They should definitely put him because FEA is the most recent FE game in the FE series.
Ryu (Street Fighter) I don't see why they shouldn't put him in this game. He has one of the most popular fighting games ever! However it is highly likely they may not put him in.
Shadow the Hedgehog (Sonic games) If possible. He was an assist trophy in SSBB and would like to see him playable since he is just brilliant.
Just my opinion, but will highly unlikely happen.
Mii (your nintendo avatar obviously
) It would be cool if you could customise your Mii by giving him different moves from the other characters in the game, and decide what costume he should wear. (such as Mario, Link, etc.)
Return characters:
Lucario (Pokemon) Even though I'm not that much into Pokemon, he was a bad@$$ in SSBB and would like to see him return.
Ike (Fire Emblem) He was a bad@$$ in SSBB as well and would like to see him return, but it is more likely they would just replace him with Chrom.
Meta Knight (Kirby series) Cool character in SSBB.
Captain Falcon (F-Zero)
FALCON PUUUUUNCH!!!
Sonic (Sonic games) He was totally successful in SSBB, and he also made SSBB itself successful in a way since he is so popular. Just think about it. Mario vs. Megaman vs. Sonic equals
ULTIMATE BATTLE!!!