Mario: Change FLUDD. I think his tornado is good as his dair, so, something else. Definitely needs a better final smash, but I can't think of anything.
Luigi: Poltergust, as whatever. Preferably as his down-b, so his tornado leaves. I'm good with his final smash, but something better than that is always welcome.
Bowser - Give him a new down+B since the hip drop was first Yoshi's.
Link: Either better bow, or better boomerang. His final smash is also good, but something more of a reference to a game.
Toon Link: Own moveset based on WW, PH and SW and maybe Minish Cap.
Samus: Her final smash not forcing her to go into ZSS.
ZSS: Different down-b, but something that can still be used as an alternative for recovery. Different final smash.
Zelda/Sheik: Make them separate characters, giving them both a new down-b, and a new final smash for Sheik.
Ganondorf: Own moveset. Only magic, only power or both. I'd be thankful for anything.
Captain Falcon: Better final smash. Can be the same one, but with more power.
Wario: More realiable up-b as recovery.
Jigglypuff: I'm ok with its moveset, but now it's a Fairy type, might throw in something. Different final smash?
Lucas/Ness: Never played Mother 2 nor Mother 3, but something to differentiate them.
Fox/Falco/Wolf: Fox and Falco are good as they're, but a different moveset would be very appreciated. Wolf simply needs to be totally different. Also different final smashes for all 3.
I think that's all.