Phil Cobb, "a small-time crook with big ideas," comes to Gotham to make it big. When he tries to put a gang together, he's laughed at because he's a nobody. Shortly after, he spots the bat signal in the night sky and is inspired to become a themed villain like the Joker and the Penguin, and to use signals as his theme, which is a pretty lame theme if you ask me. Signs and indicators and objects used in symbolic gestures. He leaves signal-based clues that will tell Batman exactly what he's up to. That's about all I know about the Signalman, but wikpedia seems to indicate that he's become a joke among the DC Universe's supervillains.
Pros: None whatsoever.
Cons: The signal clues just makes him a third-rate Riddler.
How I'd do it: I just plain wouldn't.
Casting Guesses: Ashton Kutcher. I don't like him, so the idea of inflicting a terrible role on him appeals to me.