I thought of this a few weeks ago, but I didn't want to resurrect this topic again. However, since someone else brought it back...
In Melee (and maybe the original), you know when you get knocked off the platform and you can't control your character for a bit while you're flying through the air. What if, in team, you could grab your teammate in the air and throw him back on the platform. Adds a bit more strategy to team matches.
Also, I kind of hope Sonic, Nights, Ryu Hayabusa (Ninja Gaiden), KOS-MOS make it. They've all made news recently on Nintendo platforms so it'd be nice to see them in. I'm not giving up on Belmont though!
This is kind of minor: I hope they redesign Samus to match her Prime 3 suit. I just like it more. They re-did her suit to match Retro's designs for Hunters so you never know.