According to Variety, Dinklage will play the “main antagonist”. I’m not sure if that means he’ll be the only bad guy, but all of the other X-Men films have had a lead bad guy and then powerful underlings (even if they weren’t acting under their own volition like in X2). At the very least, we know that the Sentinels (giant robots who exterminate mutants) will be a “big feature” of the story.
So who will Dinklage play? Devin Faraci over at Badass Digest made the interesting argument that the villain could be the bizarre Mr. Sinister. Even though Sinister doesn’t factor into the Days of Future Past comic book, Singer isn’t being precious with the material. I think if we did see Mr. Sinister, it would be so far removed from what we’ve seen in the comics that it would render the character nearly unrecognizable. He’s simply too outlandish to bring to the screen even if you did motion-capture. Perhaps, as Devin argues, Sinister could be a neat stepping stone to Apocalypse, but I have a different theory.
I think Dinklage will play Bolivar Trask, the creator of the Sentinels. I believe the Sentinels will be the major foe because we can only see mutants fighting mutants so many times. Furthermore, the sentinels lack personalities, so it’s up to a great actor like Dinklage to provide the emotional aspect of the threat. It would also be a nice touch that if Trask was Dinklage’s size (he’s not in the comics), then he would devote his life to creating gigantic machines as a way to realize the physical strength and power he lacks.