This list will be constantly getting updated and such. So keep an eye on this topic!
I can confirm if this game does become worked on, these characters WILL be appearing no matter what:
Super Mario Universe:
-Mario
-Luigi
-Peach
-Bowser
-Koopa Bros.{NC}
Legend Of Zelda Universe:
-Link
-Zelda (Shiek)
-Gannondorf (Toon Gannon + Demise alt.)
Star Fox Universe:
-Fox
-Falco
-Wolf
-Leon {NC}
-Krystal {NC}
F-Zero Universe:
-Captain Falcon
Earthbound Universe:
-Ness
Metroid Universe:
-Samus
Pokemon Universe:
-Pikachu
-Jigglypuff
-Mewtwo
-Red {NC}
-Green {NC}
-Gardevior - (With a Gallade Alt) {NC}
DK Universe:
-Donkey kong
Yoshi Universe:
-Yoshi
-Kamek [NC]
Kirby Universe:
-Kirby
-Metaknight
-Minion Trio (Bio Spark, Burning Leo, Sir Kibble) {N.C}
-King Dedede
Wario Universe:
-Wario
Sonic Universe:
-Sonic
-Eggman {NC}
-Tails. {NC}
Megaman Universe:
Zero {NC}
Protoman {NC}
Aile M:ZX {NC}