Bane would be perfect. I know people are afraid of using characters from the Michael Keaton - Val Kilmer - George Clooney series, but screw that, Nolan isn't.
It should be Bane and Black Mask vs Batman. And it shouldn't be Robin, it should Batgirl. Introduce Barbara Gordon. It could work very easily, I think, just have her come back from college or university or whatever, and be inspired by Batman and her Father. Bane should definitely break the Bat, which forces him to rely on Batgirl, overcoming any earlier opposition to her aid (and teaching him that Batman can't always work alone, something already known to the comics).
Also, it sets up nicely a fourth film, in which you could introduce Harley Quinn and the Joker again (now given a suitable waiting period following Ledger's death for a new actor to play him) which is one of the most twisted relationships in comics, so that they could cripple Barbara Gordon.
You can't do Clayface for the Third Batman movie. Otherwise you'll draw comparisons to Spiderman 3 and Sandman.