- Catwoman: Yeah, we’ve all been talking about her forever. Depending on the shape of The Dark Knight Rises she could make perfect sense, and having her as a potential love interest would create a love triangle with that other leading female role - which is essentially the only reason I could imagine Nolan bothering to bring in another love interest (Vicki Vale, anyone?).
- Poison Ivy: This seems very unlikely indeed. Her powers and abilities are a little too weird science for a Christopher Nolan film. Which is too bad, because Gotham City has always been the ultimate industrialized urban environment and the idea of a mad environmental crusader attacking that infrastructure has promise.
- Talia Al Ghul: If I were a betting man this is where I would place my money. The daughter of Ras al Ghul, the villain of Batman Begins, Talia makes for a nice bookend to Nolan’s films. And there are a lot of options here - maybe Talia is working to rehabilitate the League of Shadows, and so she comes to town to take down the rogue Batman. Or maybe this is the chance for Batman to show how much he’s learned since his last tangle with the group. At any rate, this is another character who offers a love triangle concept for Nolan to play with - in the comics she actually bore Batman’s baby!
It’s also worth keeping in mind that it’s unlikely that the female villain is the only antagonist in the film. While Nolan has ruled out The Riddler, I would bet that some other mob boss type villain shows up in the film as well; I kind of love the idea of Scarface and the Ventriloquist taking over the shambles of Gotham’s underworld, making the city even weirder than it ever was.