Actually, Riddler would be the most REAL. I think, for this series, that guys like Mr. Freeze or Clayface would be to science-fictiony, and I believe Nolan himself said that he'd never use the Penguin.
I can see them using Riddler, Hush (that would be interesting), MAYBE Poison Ivy if they didn't use the whole plant-controlling aspect of her character). I don't think they'd do Bane or the Knightfall storyline, because this is a 'younger' Batman, and it would be too early in his carrer.