Sims plays with it too, plays up the "dark mirror to Batman" aspect that is really at the core of (nearly) all the villains. He adds that Bane, like Doomsday, was created for one specific moment and nobody's really known what to do with him since (though Simone's done a good job reinventing him), but of course that works fine in a movie.
Course, the big question is whether he'll break Batman's back. Seems like if he didn't, it would be like on Smallville where they built Doomsday up for an entire season only to have him kill...Jimmy, and then go away forever and never be heard from again. (I think. That's when I quit watching.) But on the other hand, how do you resolve that in a 2.5-hour movie?
Somebody talked about the potential of the third Nolan movie as an adaptation of Knightfall back when it came out. But I don't really see Nolan using Robin or Azrael. Aside from that we're left with the question of how Bruce recovers, and while it could make for some great drama seeing him push through the pain to victory, months of intensive physical therapy wouldn't make for a very good montage scene.
Dark Knight DID already give us the precedent that there are other dudes going out and dressing up like Batman. I could see doing a DKR-style thing where Batman attempts to train a team of successors, and that would parallel the original Knightfall story somewhat. Hell, the "Batman is replaced by someone who becomes corrupt" element of Knightfall would work well here, I just don't think Valley is a character worth pulling out of storage to do it.
Or -- hm. Selina (mostly) gives up her life of crime to fill the void left by Batman and becomes a heroine to help bring Bane down.
You know, that sounds pretty plausible to me.