He's a psychotic genius. Admittedly unpowered, but he doesn't need them to be incredibly dangerous. What makes him a threat is the fact that his methods, objectives, even his entire personality, is just as fluid as his history. He could just decide to steal the shoes of famous clowns one week, and poison the city water supply the next one. Only a few things ever stay the same: Everything is done to lead up to to what he sees as a punchline, he's still murderous no matter what personality he's using, and he's extremely obsessed with Batman. Without him? He might try and fill that void by finding someone else to match wits with,