Bane is a strong contender, since they literally had Battison use some form of Venom to fukk that one guy up.
I want to see a grounded Nolan/Battinson version of Freeze, Ivy, or Clayface.
Man-Bat could be some sort of copy cat vigilante trying to be Batman after getting inspired by him. Think hockey pads gang from TDK, but way more violent.
I want to think you can have R'as as a mentor type that teaches Battinson some more tricks of the trade, as this Batman doesn't feel like someone who went through training with the League.
Scarecrow is another good one too. Cillian Murphy killed that role and the design was great. Just a dude in a suit, a burlap sack mask, and some drugs.
Best thing about this universe is that these characters can exist without being the plot device of the movie. Joker, Penguin, and a lot of the mob are just living in the universe without being the monster of the week. Leaves room to have Dent and the Court od Owls to develop behind the scenes.