We are in agreement that Catwoman is probably the best choice. She has mainstream appeal and isn't "too unrealistic" for Nolan and co. to use. The Riddler wouldn't be bad, either.

But, I also think that they could bring back Two-Face. I think I read one of the producers saying that his dead/alive status was left ambiguous just in case they decided to bring him back in the third film. If nothing else, it would be great to have a sub-plot of him pushing the Batman and Gordon to the breaking point in trying to keep the wool over the people of Gotham's eyes.