Black Mask, seems the most probable since he becomes the crimelord of Gotham for a short duration in the comics...

Killer Croc, maybe???..never might happen but would be dope...

Catwoman and Penguin???... I'm guessing Charlise Theron/Ali Larter/ Rose McGowan/Sienna Miller/Elisa Cuthbert/Jamie King for Selena Kyle and Phillip Seymour Hoffman/ Seth Rogen as Penguin...

Poison Ivy, seems probable if their doing a Two-Face tie in movie, probably some unknown for this role...