I just thought of something. What if we can alternate players in VS. mode? Kinda of like in Subspace emissary adventure mode where we get to choose the characters we want to adventure with and pick an order for how they would appear if one of the character gets KO'ed? And maybe there will be a tag button like those Marvel vs Capcom games (I think?) but the damage percent remains in tact either for each character or if you tag out in 90% the new character will get 90% as well.
We already have Zelda/Shiek and Pokemon trainer in Brawl, so I don't think this is impossible to hope for!