Well, that's bad. If it was possible, maybe we could have already characters as Waluigi, Daisy, Dixie, King K. Rool, Midna, Skull Kid, Ridley (...xD), Wolf, Krystal, Black Shadow, Black Knight, Magnus, Ice Climbers (maybe Popo/Nana), Rayman, Inkling, X, Zero, Silver, Knuckles and so many others, hehe...
Maybe it's possible with Assist Trophies? I remember a Elec Man over Mega Man mod and an Ashley over Lucas mod, but I don't know the way to do that. It would be cool to see a few more like Lyn over Lucina, for example.