I saw alts were mentioned.
Jr, Mac, & Wario don't really "Need" alts. It'd be cool if maybe each of the Koopa Kids had a bonus color, and I'd like to see eight for each Wario. Mac is perfect as is.
Mario: Sunshine Shirt & Shades.
Luigi: Mr. L.
Peach: More detailed and fleshed out Daisy look. Shadow Queen as well.
Bowser: Dry Bowser. Dark Bowser.
Rosaluma: Cosmic Spirit.
Yoshi: Yarn Yoshi and Boshi.
Donkey Kong: Boxing Gloves, Jr.'s shirt.
Diddy Kong: Mario Strikers outfit.
Ganondorf: Demise.
Toon Link: Pajamas outfit.
Samus: More color variation with suits.
Zero Suit Samus: Justin Bailey.
Pikachu: Ash's hat.
Charizard: Armored Charizard.
Jigglypuff: Give her the headband back.
Mewtwo: Armor, obviously
Kirby: More palettes. Brown, Emerald, Light Purple, Pale Green, Ivory, Deep Blue, Black with orange shoes, etc.
King Dedede: Masked Dedede, Samurai Costume ala PM.
Meta Knight: Galacta Knight.
Ness: Pajamas outfit.
Captain Falcon: Blood Falcon outfit and Black Shadow colors. Both would change the color of Falcon's effects to black and dark red, giving it a more "evil" feel.
Mr. Game & Watch: Like with Kirby, more colors. Ivory, Brown, Pink, Purple, Orange, Bright Yellow, Cyan, etc.
R.O.B: PM-style console based alternates. Mario Kart and Wii looks, Gamecube colorset, etc.
Duck Hunt: 8-bit alternates.
Pac-Man: Mrs. Pac Man.
Ryu: Ken.
Mega Man: All the alt types seen in MM's Final Smash.
Sonic: Recolors based on characters from the series. Red for Knuckles, Black for Shadow, Pink for Amy, Light orange for Tails, etc. Maybe add a few details to each swap that make the references more pronounced.
Project M had it right with a lot of alts, but there's still a ton more I'd like to see done. IDK why nobody has thought of dark purple/black Gamecube color set for R.O.B. I'd main that forever.