I don't think Justice League would translate to a movie as well as the Avengers did. The JLA's main characters are (generally) more conflicted and dark than the Avengers'. There wouldn't be enough time to flesh it out properly in a reasonably amount of time. JLA may tell deeper stories, but the Avengers is more fun, and it is much easier to translate fun than it is to translate depth.

Plus the main villain would have to be Darkseid because of Superman making pretty much every other translatable villain a non-threat, and there is no way they are getting that right. (and by translatable I mean: Injustice League/Legion of Doom/Whatever would take to long to develop on its own to use that, Crisis stuff no way, etc...). The only other things that could work would be Tower of Babel (but only if everyone is established very well beforehand) or Prometheus (which is what I'd want to see, but he may be too obscure/unpopular).