We've all heard the rumors about Darkseid being the first Justice League movie villain. Reactions were mixed. Personally, I think that it could work.
The threat has to be BIG enough to bring these heroes together, but it also has to be credible.
If you're not a fan of the Darkseid rumor, could you see the Anti-Monitor as the 1st Justice League movie villain? Then Darkseid could be saved for later...
He's some more info on the Anti-Monitor from wikipedia:
The Anti-Monitor is an enemy not only of the Green Lantern Corps, but also of the entire Multiverse. Billions of years ago, on the planet Maltus, there existed a race of blue-skinned and immortal beings later to be called the Guardians of the Universe. One of these beings, Krona, was a scientist obsessed with the origins of the universe, even though a taboo existed amongst his people concerning the viewing of the origins of the universe.
Krona created a machine that allowed him to see into the moment of creation. Somehow, his experiment disrupted the process of creation, with terrible consequences. The exact consequences have been rewritten over the years; originally, it was the unleashing of evil itself in the universe; later, it was assumed to be the creation of the evil antimatter universe of Qward.
During the Crisis on Infinite Earths, it was also revealed to be the cause of the existence of all parallel universes in the Multiverse; several villains were sent back in time to stop him, but were defeated by Krona and the other Oans. In a final revision, it was established that it increased entropy in the universe, shortening its existence by a billion years (see heat death). In any event, two beings were created, one on the moon of Oa and the other on the moon of Qward. On the moon of Oa, the being known as the Monitor was instantly aware of his counterpart, the Anti-Monitor (although his official name is the Monitor (of his universe), and he is often addressed as such, the name Anti-Monitor is used to distinguish him from his heroic positive matter counterpart). By this time the Anti-Monitor had quickly conquered Qward, as well as the rest of the Antimatter universe. In searching for other places to conquer, he also became aware of his counterpart. These two beings battled for a million years, unleashing great powers against each other, but to no avail. At the end of their stalemate, they simultaneously attacked one another, rendering both inert for nine billion years.
In more modern times, the being known as Pariah performed an experiment similar to the one Krona attempted long ago on a parallel Earth (this was changed later to an alternate world in the post-Crisis single universe). This experiment resulted in the reawakening of both the Monitor and the Anti-Monitor, and the destruction of Pariah's Earth and his universe. The Anti-Monitor rebuilt his army, taking over Qward and using the Thunderers as his own private army, as well as creating the Shadow Demons from the elite of the Thunderers.
The Anti-Monitor then released a massive antimatter wave, absorbing the energies of the destroyed positive matter universes and growing stronger even as his counterpart grew weaker, and employed the second Psycho-Pirate, using his emotion control powers to terrorize the populations of the planets he sought to conquer and destroy. The Monitor, along with his aide Harbinger, gathered a group of heroes and villains from various alternate universes in order to combat the threat of the Anti-Monitor.