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!
I'd mark out for that or Ten Nights of the Beast (as unlikely as it may seem right now).
