Keep in mind that its early yet, and given that Justice League doesn't have a director, so these things can change. And that's if this rumor turns out to be true, of which there's no guarantee. That said, this comes from Latino Review, a site that's generally been Johnny-on-the-spot with these kinds of stories and with Justice League in general.
According to El Mayimbe at LR, the Justice League will consist of Superman, Batman, Wonder Woman, The Flash, and Green Lantern.
Pretty much exactly the line-up you'd expect. These five are more or less DC's most recognizable spandex-wearing neon gods. Not only are they the most marketable, then, they're also the most efficient from a story perspective. Unlike The Avengers, which Marvel Studios teed up with individual features, Justice League is going to have to do some narrative heavy lifting to establish everyone while also telling a compelling story that brings them all together satisfyingly. With these five heroes, mass audiences are familiar enough that no massive exposition dumps will be necessary to introduce them.
In addition to the main crew, there may well be room for one or two cameos from lesser-known Leaguers, with Martain Manhunter, Aquaman, and Hawkeye listed as the most likely possibilities.
The roster rumor from earlier this month, which also included some rumor nuggets on the story, implied that Martian Manhunter would be a member of the League and that Aquaman would play "a key role in the story."
Gangster Squad writer Will Beall is currently working on the screenplay. The story is said to be inspired by Justice League of America issues 183-185, which find a pre-Crisis Justice League and Justice Society facing off against interstellar despot Darkseid when he attempts to nudge Earth out of orbit, making room for his homeworld of Apokolips.
Again, though, that's scuttlebutt for now, since Warner Bros. isn't saying a peep about Justice League officially. If this line-up of heroes turns out to be accurate, it still doesn't answer just who will be playing these characters. It's been implied on more than occasion that Henry Cavill will reprise his Man of Steel role in Justice League, and a rumor shot down by the actor's representatives claimed that Joseph Gordon-Levitt would be under Batman's cowl as John Blake, his character from The Dark Knight Rises. And what of Ryan Reynolds, who played Hal Jordan in the not-too-well-received Green Lantern in the summer of 2011? Will Justice League have a different character wearing the ring, or is Reynolds getting another crack at the character?
We just don't know.