Will Beall has been working on the screenplay for Justice League since the beginning of the year, and apparently Latino Review knows someone close enough to him to have gotten their hands on a few of the pages. They reveal that the villain will be Darkseid, the supreme monarch of the planet Apokolips and the greatest threat to the DC Universe.
This is DC Comics' official description of the infamous character.
"Darkseid is the ruler of Apokolips and Darkseid's Elite and was a major player in the cosmic war that was the Final Crisis. Unto the hell that is Apokolips came Darkseid, malice personified, a merciless tyrant who demanded unwavering devotion and abject fear from all his subjects. A being of unparalleled strength, Darkseid nevertheless preferred not to sully his gloves with combat unless provoked to action. His eyes emitted the formidable Omega Sanction, ray beams that could disintegrate, teleport, or resurrect depending on the dread lord's wishes. His sole mission in life was to find and gain control over the Anti-Life Equation which would give him the means to usurp all free will in the sentient universe. Darkseid first appeared in Superman's Pal, Jimmy Olsen #134 (December, 1970), and was created by Jack Kirby."
At this time, Justice League does not have a set director or any cast members lined up. Warner Bros. is waiting for Man of Steel to arrive in theaters this June, to see how it fares, before moving full steam ahead on this potential blockbuster, which is scheduled for the summer of 2015. That will be the same summer as The Avengers 2 arrives in theaters.
Some folks are speculating that Darkseid will actually make an appearance at the end of Man of Steel, the same way Thanos appeared at the end of Marvel's The Avengers. Those same fans are complaining that Darkseid is a little too close to Thanos in his overall character arc. They also feel that the villain is too big to set up in a first film, which will also be introducing most of its superheroes on screen for the first time.