FirstShowing.net has a real doozy with rumor that Michael Fassbender will be Apocalypse. According to various sources and scripts, at the end of X-Men: Days Of Future Past, Fassbender sees something fall from the sky where he encounters an alien that merges with him.
...he encounters what is described as an "alien form" who is "towering, terrifying, with a twisted metal face and dark pits for eyes." It's Apocalypse and he says, "I have crossed rivers of time to find you." Magneto asks, "Who are you?"
The alien responds, "I am your future." Suddenly Apocalypse merges with Magneto, who screams and blasts an insane amount of energy and shockwaves around him. This energy explosion levels everything around him, with a blast radius that goes for miles. As Magneto looks at what he's done, he has a realization of the power he now holds and a villainous grin begins to form before the film cuts to black.
This early on, it remains to be seen whether the rumor holds true or if Fox is intentionally putting out a feeler, which sounds more akin to the comic book version of Onslaught (merging of Magneto and Xavier into all powerful being).
Personally, this doesn't sound all that appealing.