The mobsters he's used so far are pretty boring. Penguin's connections to both the mob types and the supervillian types would be the bridge between those two areas of the filmverse.
Although my #1 choice would be Phantasm overall, but as I said she'd have to be working for someone else.
Well, I've heard Catwoman's in it? Well, I would like Poison Ivy and maybe Clayface, Theres rumours Riddlers the main character, Joker can never come back... ( R.I.P. )
maybe bane to tackle the issues of false imprisonment and steroid abuse
Mega Kicks for all Who defy the Emperor