Kinda surprised someone beat me to Catwoman. I really think she could use her own game where she steals, and defeats security systems, and such.
I wouldn't complain if she were teamed up with Harley Quinn, and Poison Ivy in a Gotham City Sirens game. Maybe the plot would be to work with/against other criminals to not be the nail that stick up too much or else Batman comes to kick your face in, while amassing as much wealth as possible? maybe to upgrade your home/base, gear, maintain enough funds to keep getting more materials, etc.
Really surprised someone beat me to Secret Six. I definitely agree with that.
Birds of Prey could work, too.
Batgirl, maybe. "Which one?" would be the big questions as each are pretty distinct. Robin got his own DLC in Arkham City that didn't have batman butting in.
I could definitely be happy if Wonderwoman got her own game.
Batman Inc. maybe. Instead of focusing on just batman, maybe the rest of the members could be around, and useful?
How about Power Girl? I'm sure that'd get some controversy, especially if it's the pre-new 52 design, not that I'm a fan of that sort of thing. Toss in Super Girl, and I think that'd be grand.
Of course there'd no doubt be the angry folks that'd complain if either character got a good game before Superman did.
It'd be nice if things got mixed up some, and/or lesser known characters got some attention to, maybe, inspire people to be fans of them?
I mean Project X-zone had me looking up people thanks to it's massive cast that had people I didn't know much, if anything about until I played that game. Scribblenauts Unmasked did the same a bit, too.