Rayman would be a great addition for his limbless body to have great attack variety, and great recovery.
Banjo and Kazooie would function like Ice Climbers and Rosalina/Luma. You control Banjo, and without Kazooie, you're pretty much screwed.
Steve (Minecraft) would have a HUGE attack variety, but would have very low weight (hey, he's only got 10 hearts).
The Smash Bros. developers need to redo Ganondorf. He sucks. Just a Falcon clone. Not cool, Nintendo.
And finally we have Robin (Fire Emblem: Awakening) because he would have some BADASS MAGIC SKILLS.
Any other characters you want for the game, post on this thread.