I am kind of surprised no one has mentioned Apocalypse. He's easily the most interesting/powerful villain in the series. However, I don't see how they would fit him in at all.
That being said, the Sentinels are a logical next step in the progression of this storyline. Xavier nearly killing everyone on Earth would warrant the creation of a defense against the mutants. Frankly, I'll be surprised if the storywriters fashion anything but sentinels or an already-existing villain from the X-Men comics. Brian Cox did a damn good job with the role he had, but he was completely made up.