Everything has been unlocked. All the characters, all unlockable costumes, all the original mini-games, everything for the gallery..including character profiles, endings, music, etc..There is absolutely nothing left to unlock. Enjoy!
Or when your player dies, you can say in code, if enemy exists, do nothing, but if enemy does not exist, then create enemy at certain spot. (This would be a pain tho for lots of emeies)