We have to remember Christopher Nolan is the king of misdirection. You see it in his movies and you hear (or in some case not hear because he does not like to divulge anything about his movie) it in the news.
Think of this Bane or Catwoman do you really think they really pose a threat to Batman or Gotham. Bane is not too recognizable to a common movie goer to think much of him as the main villain (He is smart and he is strong but not that iconic, not in the level or near the level of Joker). Catwoman, if ever, does not cause that much of a threat considering the one made by Ra's (Burning Gotham to the ground - figuratively) and Joker (Making Chaos).
So, I think you need a masterplanner. Someone who can orchestrate some clever mayhem. You could have Bane or Catwoman as the moving force or pawns but in the end you need a big reveal and that is Joseph Gordon Levitt as the RIDDLER (Who can create a lot of confusion).
Nolan can also go in this direction, though many may react badly to this, that this version of the RIDDLER can be related to the JOKER (explains the uncanny resemblance of Levitt with Ledger).
I found this old poster that might prove my point. I "think" its fanmade but it has not yet been proven beyond reasonable doubt, so this could be legit(?). But if this turns out legit, with its clever use of a Jokerman font for a question mark, then you heard it here we might be having RIDDLER as the BATMAN's main nemesis on DARK KNIGHT RISES.
By the way, I think Eckhart will be back as Two-face. I still have a feeling that he did not die on the last one. It seems like a fake death (like what they did with Gordon).