At this point, WB/DC may have to do it in reverse... which is develop a JLA film first, then spin potential individual franchises from that depending on how well it does. And if it's done right it would be absolutely huge, as in AVENGERS huge or even bigger. You're talking about Batman, Superman, Wonder Woman, Green Lantern, and The Flash. These are all elite, top-tier comic characters. And it doesn't get any bigger than Bats and Supes.
The problem is they have no cohesive vision. They need someone like Marvel has in Kevin Feige to take the reigns and shephard it all. Nolan is not that person, because he shows an open distance from true comic book sensibilities. And I hope it isn't someone like Geoff Johns or Jim Lee, either... That would set up a total cluster____.
WB/DC is in a weird place with this right now. The Nolan Batman films are clearly not meant to take place in any shared DC film universe, the GL movie completely underwhelmed, Flash & Wonder Woman (once a Joss Whedon project) languish in development hell, and they're about to launch a Superman reboot produced by Nolan with a 3-picture deal with no indication of whether or not that could be part of a bigger DCU. Obviously, a successful Supes film would be the perfect kick-off and jump starter, but it's all an unknown. Like some have said, they seem to have no plan when it comes to a consolidated DC movie universe. And that's a damn shame, because unlike Marvel, DC controls the rights to ALL of their comic characters via WB.