Don't know if he needs to be playable, but having to fight the Frankenstein gardener from Legacy of Darkness as a "stage hazard" in Judgment would have been fun. Imagine having to fight him in a courtyard lined with hedges with a horde of zombies or other lesser creatures. You could knock him out, but only for a short while, while you concentrate on killing the other creatures in the area. He then resuscitates and charges at you like a madman with his chainsaw, forcing you to turn your attention back to him. The stage would clear when you've defeated all the zombies.