If Mewtwo isn't in, well I'm just going to mention my Pokémon options.
First off its not too bad, I would have Charizard to work with, new and improved without that stupid trainer and pathetic Squirtle and Ivysaur hindering him, and that new move he has looks very nice, Greninja would be fresh, hopefully he'd get a Mewtwo pallet swap if Mewtwo wasn't in, and then Lucario, he sucked in Brawl, but he's worth another shot, and hopefully he gets a Mewtwo skin keep in mind Mega Mewtwo X is psychic and fighting, so it would actually make sense for Mewtwo to be fighting like Lucario, maybe the final smash could be a skin of Mega Mewtwo X.
As for Jigglypuff, I'm not play as this thing because its the main thing responsible for keeping Mewtwo out, I see no reason why all characters from Super Smash Bros 64 have to be included anyway, that game