since the first dark night focused more on the realistic side of batman, and more psychological villains i think a couple of these guys could show up:
Deadshot - he could be some crazy serial killer that batman needs to find, maybe even as just an underlying villain to the plot
Ra's al Ghul - hes a terrorist and he knows batmans real identity. could be some crazy twists there
Joe Chill - he killed batmans parents! he was hired by Lew Moxon, who could show up too, it would be interesting to see christopher nolan dig into batmans past in this movie
im guessing they could choose alot of different villains, the riddler seems most likely to me. but i think any of these ones would be really interesting, especially joe chill
Chris Nolan confirmed that there's no Riddler.
I can see Deadshot as some kind of henchmen that's hired by the main bad guy, but i dont think he could carry the thing by himself.
I wouldn't mind Ras Al Ghul again. they killed of Liam Neeson in the first one, then we're supposed to believe that the League of Shadows, this group that's existed for thousands of years just disbanded and quit? wouldn't they want some kind of revenge on Batman? i feel like they just moved on from that to the joker and forgot about all that stuff. it'd be a nice way to tie the third movie back to the first one.