I would put in lets say 1 from each franchise and have the game style change between matches. each match would consist of a smash type round and an allstars type round and then the kill/death ratio is calculated accordinly.I would have to keep the idea of mashup stages but this time have either nintendo worlds invading a sony world or vice versa.
For the characters I would choose
8. donkey kong
11. captain falcon
6. sir dan