Maybe it's too soon now, but I'd say that DC has the best shot of getting their characters all in one movie. Other than Batman and Superman, all of their characters are under the DC license. Sups and Bat are both under WB. If they could sit up the Man of Steel to just introduce the idea that he is not enough to defend the planet, that's all it takes. Just a line would send fanboys and entertainment media through the roof with the possibilities. Batman does not need a movie to work him in, I dont feel that the Flash or Wonder Woman would do good in their on movie (Quick, name an awesome WW villain that would not look cheesy on the big screen?).
I would recommend that the movie just brings the heroes together but not under the JLA title until the end. The world is in the fight of it's life against an invasion of enemy robots. Heroes all over the world are holding their own, then darksied pops up (?). Cities are falling and the heroes realize they cannot win this war without leaving the world in ruins, so they head off to apocalypse. There, they do there stuff, win and return home to celebration. They decide that they will form a earth defense force in case this ever happens again. Pan out to above the earth, Brainiac. He's the one who started the initial invasion to lure Darksied to Earth. This was some masterful plan of his to defeat DS, distract the heroes of earth while he builds something and leaves a power vacuum for Brainiac to fill. The sequel will deal with Brainiac, obviously.
And it should be in the dark style of the DC universe's cinematic.
How would you want to see the JLA's on the big screen?