Man, so many things to talk about...
As far as characters... Well, let's see...
Playable characters, I'd like to see Megaman, as it says in the poll up there. That could actually work. I'd also like to see Lyn become promoted from Assist Trophy to Playable Character and Roy make a comeback (both of them are from Fire Emblem).
Starfy would be a lot of fun, as well. Paper Mario, too.
Then, there's always revisions. I'd love to see Donkey Kong use his Coconut Cannon and Orange Grenades from Donkey Kong 64. After all, if Diddy Kong got his Peanut Popgun and Rocket Barrel Pack in Brawl, why can't Donkey Kong get some of this cool stuff, too? I'd also like to see characters with similar movesets get some tweaks. Mario & Luigi are starting to differentiate. But, you know what? I think Mario should have his Spin Attack again and Luigi can use the Pollygust in much the same way as Mario used the FLUDD in Brawl. Gannondorf has so much potential for such completely different attacks and I should even need to bring up the StarFox problem...
As for stages... Well, I think we're long past due for Bowser's Castle. I mean, seriously. That should've been in the first game. We're at three and still no Bowser's Castle. Coming from the Sonic series, we could really use Angel Island. It fits Super Smash Bros. so well, as it's already floating in mid air. We could also really use The Moon from The Legend of Zelda: Majora's Mask.
Oh, and I'd also like to see Back Special Moves, if that's ok.