Man, most of these suggestions on this page are completely abhorrent.
I say we ditch Falco and Marth, make Ike faster (But weaker), give Ganon the f*cking sword, declone Toon Link, Luigi, and Lucas, and add Bowser Jr, K. Rool, Sylux, Ridley, Kamek, Shy Guy, Adeline, Krystal, Darkrai, Mewtwo, make a new Pokemon Trainer replacing the old one (With Turtwig, Monferno, and Empoleon.), include Black Shadow, Micaiah, Black Knight, Claus, Bomberman, Megaman, and Mii's.
There's more, just not at the top of my head...
ORITE STAGES:
Bowser's Castle
Ricco Harbor
Good Egg Galaxy
Luigi Circuit
DK Island
Kakariko Village
Death Mountain
Ganon's Castle
Phaaze
Cracko's Lair
(Redone) Fountain of Dreams
(Redone) Venom
Fortuna
Sargasso Hideout
Temporal Tower (Yes, the one from Pokemon Mystery Dungeon 2.)
Sinnoh Gym Tour (A stage which randomly switches to each different Sinnoh gym, with their own special qualities.)
Gallian Border
Shadow Dragon's Castle
Tazmily Village
Sunflower Field
Retro Climbers (The 8-bit Ice Climbers game! Moving stage, though, but breakable block platforms.)
Gyromite (Basically a huge spinning gyromite...)
Fire Field
Death Wind
Valley of Repose
Bulbax's Cave
Bomb Zone
Dr Light's Lab