Well it's been announced. After the Red Hood animated movie gets released, the next one is already in development. It will be the sequel to the Batman/Superman: Public Enemies storyline "Batman/Superman: Apocalypse."
The storyline itself is VERY larger than life, with Bats/Superman/Wonderwoman all going to apocalypse to get supergirl back. Hopefully it will capture some of the awesome batman moments and cut out some of the supergirl shopping and Bats/Sups vs Wonderwoman plot lines which felt a bit out of place.