My opinion [Dark Knight SPOILERS ahead]
Well, IIRC, Nolan specifically mentioned in an interview that he'd stay away from the more sci-fi villains of Batman's gallery (ala Mr. Freeze, Clayface, & the like)—not sure if the poison-immune Poison Ivy would work in that regard (can't recall where; sorry—I'd provide a link if I could recall the source).
I'm guessing that the Riddler would be the next villain, because:
- He's one of Batman's big-name villains;
- He's obviously not a sci-fi based villain;
- His sort of crime wave would allow Batman to show off his intellect & detective skills (even moreso than in The Dark Knight); and
- His schtick could easily blend into a bigger theme that is present throughout the movie (ala the "fear" theme of the 1st film and the "choice; good/evil or order/chaos" theme of the 2nd film). Perhaps something like power & responsibility (Batman using his talents for good, while Riddler uses his talents for crime), the thrill of the challenge, identity (Batman trying to deduce Riddler's identity while the police are trying to deduce his), or the like.
The last point could easily apply to Catwoman as well. Then again, I could be way off-base with this (I was with Dark Knight—I thought the big theme of that was going to be
sanity, given the 2 villains involved).
I have strong doubts about
Penguin (he seems like he'd be a background character criminal in Nolan's style of films, like the Chechen or Sal Maroni in
The Dark Knight). Maybe
Black Mask (who Nolan has hinted at before).
Talia al Ghul could be a possibility, and it'd tie it to the first movie as well.
Deadshot (as an assassin) or
Firefly (as an arsonist) may be a possibility as well. Catman, Killer Moth, and other anti-Batman sorts seem a bit unlikely.