Though Justice League does not yet have a director, Gangster Squad writer Will Beall is hard at work on the screenplay, and Warner Bros. is looking to have the film in theaters in 2015, not coincidentally the same summer that a Joss Whedon-directed The Avengers sequel is scheduled to arrive in theaters. A mid-credits scene in The Avengers indicated that the sequel's supervillain will be the cosmic conqueror Thanos.
If the rumor we're hearing now turns out to be true, then Justice League will perhaps unsurprisingly feature a similar sort of bad guy. If you consider the villain's identity a spoiler, then turn away now, for there are potential spoilers right here.
According to Latino-Review, a site with a hell of a track record getting the drop on comic book movie exclusives, the Justice League bad guy is none other than Darkseid.
Darkseid doesn't have the pop culture profile of a Lex Luthor or a Joker, but he's a heavy hitter in the DC universe. Created by the truly great Jack Kirby when he first flipped Marvel a little bird and skipped over to DC as part of Kirby's inexplicably not-hugely-popular Fourth World Saga, Darkseid is the supreme ruler of the planet Apokolips. He's a New God, a ruthless, impossibly powerful intergalactic overlord in search of the Anti-Life Equation. He's probably the biggest single bad guy in DC's stable, so it makes sense that Warner Bros. would call on him to be the antagonist in Justice League.
Again, though, this is an unconfirmed rumor, albeit from a source most definitely worth trusting.
Rumors are really all we have to go as far as Justice League is concerned. The general impression is that the studio is whipping this one together post haste, trying to finally exploit some of the crazy recognizable characters in its DC stable onscreen. Unlike Marvel Studios, which had five movies to cross-pollinate, establish characters, and build up to The Avengers, Warner Bros. doesn't have too much time. There have been indications that next summer's Superman reboot Man of Steel will act as a springboard for Justice League, and that makes perfect sense, but is yet another info-nugget not yet confirmed. Additionally, there was that rumor that Joseph Gordon-Levitt's John Blake would make the jump from The Dark Knight Rises sidekick to Justice League Batman. That one was denied by the actor's representatives, however.
So basically stay tuned to see if Darkseid ends up menacing Justice League when the summer of 2015 rolls around.