I'm doing a speculate. Smash Brothers in the style of The King Of Fighters, and the resulting roster.

My self imposed rules:
8 teams, one for each starting character
The hidden characters have to be incorporated into these teams somehow
Nothing from after Smash 64's NA release date of April 24, 1999.

The final boss is Rugal

Teams I have completed:

Team DK: Donkey Kong, Diddy Kong, Dixie Kong

Team Hyrule: Link (Adult), Sheik, Impa (64)

Team Star Fox: Fox, Falco, Slippy

Team PKMN: Pikachu, Jigglypuff, Eevee

incomplete:

Team Mario Bros: Mario, Luigi, ?

Team Hunters: Samus, Captain Falcon, ?

Team ?: Yoshi, Ness, ?

Team Pop-Star: Kirby, Meta Knight, ?

as a bonus: a mix-and-match of SSB and KOF teams

Team Firestarter: Mario, Luigi, Kyo Kusanagi

Team Animalistic: Donkey Kong, Robert Garcia, Ryo Sakazaki

Team Weapon Masters: Eiji Kisaragi, Billy Kane, Link

Team Heroine: Mai Shiranui, King, Samus

Team Ikari Dinos: Ralf Jones, Clark Still, Yoshi

Team Psycho-Soldier: Ness, Athena Asemiya, Kirby

Team Hunters: Fox, Captain Falcon, Terry Bogard

Team Monsters: Pikachu, Jigglypuff, Iori Yagami