I don't see why this can't be a "one-off" (pending sequels) scenario.
I think the trick is to make it J'onn's story while having him collect the group in the second act and allotting each five minutes or so of "origin" expo.
One of the advantages DC has over any of the Marvel properties is the heroes (at least the base group) are pretty much ubiquitously recognizable. The one exception is probably MM. Which is I why I think you tell the story from his perspective. Use the first film to help familiarize the audience with him.
Actually, I think it's actually better this way. The characters' respective universes are much more diverse than the Marvel heroes'. This way you can keep everyone grounded in one central world without having to be tied down to the others.
You could sill used Cavill and Reynolds (heck, even Bale if he was up to it), but only acknowledge their own worlds/films with winks and nods.