The first official plot synopsis for Matthew Vaughn's X-Men: First Class has hit the web, and there's a possibility that this synopsis reveals the main villain of the film. Spoilers!
Here's the official plot synopsis (via Screenrant):
X-Men: First Class: "Charts the epic beginning of the X-Men saga, and reveals a secret history of famous global events. Before mutants had revealed themselves to the world, and before Charles Xavier and Erik Lensherr took the names Professor X and Magneto, they were two young men discovering their powers for the first time. Not archenemies, they were instead at first the closest of friends, working together with other Mutants (some familiar, some new), to stop Armageddon. In the process, a grave rift between them opened, which began the eternal war between Magneto's Brotherhood and Professor X's X-Men."
Screenrant claims that the word "Armageddon" is a reference to the obscure character Armageddon rather than the end of the world. In Marvel continuity, Armageddon is the alternate reality "son" of Apocalypse, who grew Armageddon in a lab by mixing his and Jean Grey's DNA. He however bucks his villainous heritage and joins Professor Wolverine's X-Men (it's complicated). If he is in the film, his origin story will be pared way down.
Vaughn hasn't strayed from including no-name X-characters in First Class (see: Darwin, Azazel), but I severely doubt that he'd use Armageddon's original backstory. If Armageddon actually is the villain of the piece (and Kevin Bacon's Sebastian Shaw is just a decoy), this D-List character could be used to introduce Apocalypse in future installments of the franchise.