How about adapting the Earth One books?
They are done as stand alone Graphic Novels, so they kind of lend themselves to being adapted as stand alone films more. But would fill more screen time then The Killing Joke did.
shareSimilarly, I would like to see a straight-up adaptation of The New Teen Titans: Games, the original graphic novel by Marv Wolfman and George Perez.
"If I'm going to have a past, I prefer it to be multiple choice!" - The Joker