The Justice League movie is already going to have to deal with people calling it a copycat movie (myself included) so they'e going to want to do as much as possible. Avengers has already done aliens and will soon do Thanos. There goes the White Martians and Darksied as possible JL villians if they want to distinguish themselves from the marvel movies. So who can they kick/pow/zap that won't be totally lame? I see two possibilities. The first is to use the yellow lanterns. The yellow lanterns are usually reserved for the Green Lantern but as the Avengers movie showed us it is possible to elevate a villain usually reserved to one character (loki-thor) and expand him into a universal threat. One Lantern is already one of the most powerful beings in the universe. No take an army of them motivated by fear and it is possible to see why it would take a whole team of superheroes to fight them. Unfortunately this is probably going to be reserved for the Green Lantern 2 movie but we can see why it would work.
Another possibility I would like to see is Superman as the villain. The Avengers again did a great job (wow seems like the avengers might be a good movie) of making The Hulk seem terrifying even to other superheroes and immortals. The Hulk could have very easily been a believable villain for that movie and it would have taken all the other heroes combined strength to wield him in. But The Hulk is all rage so his character motivations would not have been very deep. The writers for the JL movie could come up with all sorts of powerful motivations for why Superman all of sudden wants to conquer the universe. Perhaps it is in an attempt to bring back Krypton, maybe he is being manipulated, who knows but if done right it could be very intriguing and possibly moving. Besides who doesn't want to see Supes and Batman get into it?
Who else could be a villian? Sound off your ideas in the comments below.