Job #44: the Main Event
Basically a halfway point between the Melee Adventure Mode, Subspace Emissary, and taking some plot from Marvel's Secret Wars. Some sort of cosmic villain is interested in all of these worlds, and pulls them out to prove who's the champion of them all, and has placed them in a gauntlet consisting of recreations of their own worlds all on one world. This is the central conceit for them fighting through levels based on their games, complete with enemies, fighting clones of the game's roster, and bosses based on those games. You can take this as us adding in more bosses, or potentially adding more villains to the roster, if you want. Then at the end, you fight through an original level that leads to the big boss, along with a potential fight against the Fighting Polygon Team before that. As far as character interactions go, you may have images and text dialogue scenes in certain situations beforehand, that sort of cutscene, if doing it the Subspace way is too involved.