What the heck are the other series that invade these levels? Some alien guy and some robot and some blimp with monsters I have no idea what's going on --- http://i.imgur.com/x569K.jpg Now's our chance for an All-Out Attack!
A better question is why did they bother with mashups in the first place?
What a waste of money and resources. They should have stuck to one theme per stage, not only could we have had more stages this way, they also could have used that time and effort to polish up the rest of the game.