Ventriloquist and Scaface: Ventriloquist has a mental problemwhich makes him believe that a puppet is telling him to do bad things. Now, judging by the Nolanverse's Joker, that would be realistic.
Penguin: A deformed mob boss with two little toe-thumbs and a hooked nose. He's also a midget. That would fit in alot! Nolan can definitely find a way to make Penguin darker.
Firefly: A pyromaniac dressed likie a bug on the run through the city laughing his head off maniacally. AWESOME!
Harley Quinn: A doctor gone insanely in love with Joker. I'm still not sure...
Any more suggestions?