The mobsters not confirmed are Sal Maroni, Rupert Thorne, Roman Sionis, Tony Zucco, Great White, Red Hood....but, look at the list:
Zucco - Can't have him without Robin
Great White - Would be a waste.
Sionis - No way.
So that leaves Maroni, Thorne, and Red Hood...obviously, Maroni has a shot since we know Two-Face's deal with him, and there's potential for the Red Hood to be involved, since its The Joker...Thorne, I can see him looking the most like, but chances are he's either one of the others two. My guess is he's Maroni, but its a bit of an odd choice for casting if you ask me.