I think it might also be Hush. So far the films have been loosely based on The Bat-Man classics: BB is Year One, DK is Killing Joke and The Long Halloween. What's next? Apparently Hush isn't that good (never read it), but it's still an important storyline. I doubt they'll go with crossovers like Knightsend or No Man's Land; it's gotta be something self-contained that Nolan can read as a "graphic novel" on the airport without being embarassed.
Whatever storyline they go with, I bet the movie will take at least a couple of plot points from DKR.
They could adapt parts of the Jim Starlin Mini series, The Cult!
