They tried to do a Justice League movie last year and it sounded like an awful idea. It was to be all CG, and introduce every character's origin in like, 120 minutes.
If they were to ever do a JLA movie (and I think that would be one insanely awesome action movie), they would REALLY benefit from doing it the way Marvel is doing it. Introduce the major players in their own movies and then have them show up in a Justice League movie, where they can introduce maybe one or two new characters (i.e. Martian Manhunter as I really don't think a movie about him would hold up, The Atom, Aquaman, etc. Basically any character that couldn't really sustain a good movie on their own).
Also, they really should not put any characters in here if they can't get the main movie's actors in it. What I mean is, if the next Batman movie is coming out and Christian Bale is Batman/Bruce Wayne, DON'T put Batman in the Justice League movie and have it be somebody else (which is what they'll have to do since Bale has said he doesn't want to do a team movie.).