First, I will predict the bad news:
I predict that there will some characters will leave. Not to start rumors, but I think Sonic and Snake will leave. However, there is a chance that the others will stay and new third party characters will join from other universes. I can also see Tabuu returning for round two, as well.
Now, for new characters.
Blaziken
Ephraim (Fire Emblem)
Plusie and Minin (Pokemon)
Roy
Mewtwo
Dr. Mario (Who saw that coming?)
Funky Kong
Nightmare
Shadow
Lyn
The Dark Knight
Dixie Kong
Louie (Pikman)
Paper Mario
Balloon Fighter
Mii
I got tons of characters, but I need to think of more...
Hey, I've got a good name too: Legends. Super Smash Bros. Legends.