I'd love to see Donald Duck in Smash. To hear his crazy screams allong with Mario's 'woo-hoos' and Yoshi's cheerfull jibberish, just his voice in the game alone would be fantastic as frick. I know, if it would be possible for classic disney characters to be in, Mickey mouse would be the obvious representative pick. But c'mon, lets be frank, everybody likes Donald better anyways.
Would be the cherry on top to have him in his Kingdom Hearts outfit as wel. Just to get the KH fanbois all riled up. Because i think Smash has plenty of angsty anime-teens already. Though I'd trade in Sora for one of the Marth clones any day, at least he would have a unique moveset, as much as I find him incredibly dull as a character personality wise. And how freaking dope would it be to have the Mario bros vs Donald and Goofy in a teambattle. (Everybody also loves Goofy more than Mickey btw).
And just like the Megaman from the original NES boxart (the one that's hilariously hideous) has made a joke appearance in a fighting game, think in a marvel vs capcom game but don't quote me on that one. Just like that version of Megaman, It would be hilarious to have Link from one of the Phillips CDI games. Would just be funny on its own to have another version of Link next to the three we already have (4 totally different swords bby!). And because its also an interresting lil bit of Nintendo's history, wich would be really cool of Nintendo to finally acknowledge it and make some fun of it in the meantime.
And for the 0% chance that there will be an actual adventure/story mode added, like the stage builder was added to the game later on (special rule matches to win stat boosting google-image-results just doesn't cut it for me, sorry), I'd like to see a story mode in the spirit of Smash 64 (back to the roots boi!). Now the OG Smash didn't have much of a theme or story going on, but the first 10 seconds of its opening sequence have a clearer and better theme and story than Ultimate has right now. Just a hand in a kid's room, messing around with toys, bringing them to life and pitting them against eachother. Wich i think is a way more untruiging theme than the abstract art appocalipse, accompanied by the poetic jibberish that is the lyrics of Ultimate's opening video.
So you have a hand that puts stuff together to play with, (bassically creating and keeping the world that makes the game itself). Now what kind of character could be a natural enemy? Winged faceless blue glowy man, static noice vomit-creature or the light at the end of a tunnel? No of course not, how would those ideas make for a fun boss character? I'm talking Master Foot here! Stomping all over the place, kicking the crap out of everything Master Hand has put together!
Maybe way too cheesy, but a lot more appealing and fun than the blandest of bland that is 'the world of light', though the opening animation looked awesome, but that's the only good thing i can say about that.
Now why this whole ramble on a different storymode? Wasn't specified that we're only talking 'playable' characters here. And I really believe a somewhat memorable boss would do the game good.