I'd make an obscenely cute Smash Bros. It would have a pretty cartoony style and some characters, like Link would have all new designs. I'd also make characters heavier in general, though everyone would be way floatier. Fighters would also appear much bigger on screen. It probably wouldn't be tournament viable at all. Most characters would be physical attackers, projectile characters like Samus would be special.
I don't care about your rules, btw. This is a fantasy:
Mario, Luigi, Peach, Yoshi, Shy Guy, Bowser, Toad, Wario, Boo, Geno, Waluigi
Link, Zelda, Ganon, Impa, Ghirahim, Groose, Midna, Deku Scrub, Moblin, Tingle, Great Fairy
Captain Falcon, Pico
Ness, Paula, Poo, Lil UFO (i know that's especially dumb and don't care)
Pikachu, Jigglypuff, Bulbasaur, Squirtle, Mew, Mewtwo, Charizard, Riolu, Sylveon, Haunter, Starmie, Gloom
DK, Diddy, Dixie, Kritter
Samus, Zeta Metroid
Kirby, Dedede, Waddle Dee, Meta Knight, Dark Matter, Marx
Fox, Wolf, Falco, Slippy
Marth, Andy, Lip, Chibi Robo, Duck Hunt, Ice Climber, Squid Kid, Marshall, Dillon, ROB, Pit, Medusa, Takamaru, Mach Racer
Goemon, Banjo-Kazooie, Marina Liteyears, Firebrand, Moogle, Slime
Ok sorry I'm done