We now know a solo Batman movie is in the works written by Ben Affleck and Geoff Johns.
Back while promoting Batman Vs. Superman in March, Affleck was asked if he was directing a solo Batman movie, would he use the stories from Batman: The Killing Joke or Red Hood. Affleck replies he would do an original story, so it would be something new for the fans, but would borrow what he could from the comics.
Here's the blurb via Cines Argentinos.
I think what I would do, what I probably would do, is in some ways what Zack did with this movie, which is to borrow certain things from the great comic books, but create an original story around it because I wouldn't want the fans to already know the story and be familiar, but I would borrow, steal, great things that have been done before in the comics.
We can obviously guess Affleck was writing the Batman movie at the time of being questioned, so his answer probably sheds light on his and Geoff Johns' direction for the movie, which is speculated to be titled The Batman.