Over at First Showing, they go more indepth with the spoiler. If you'd like to read the spoiler feel free to click the link or highlight below for the excerpt.
However, the credits bumper jumps back to the past with a young Erik Lehnsherr (Michael Fassbender) in a ramshackle house in the desert eating dinner, clearly on the run from all the authorities who want to bring him down. While using a well outside to get some water, the young Magneto sees something fall from the night sky and crash to Earth nearby. Upon inspecting the impact zone, 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.
Overall from the looks of it, it seems an X-Men: Onslaught film may not be far behind after X-Men: Apocalypse.
Many are reporting this means the entire timeline may get skewed and rewritten to write out the previous trilogy entirely. Personally, my view is that it might be more likely that this helps setup the pieces needed for an Onslaught film after X-Men: Apocalypse is resolved that may or may not also take into account the events of Last Stand.
Only time will tell, but it seems that Fox may be going full throttle on expanding the X-Franchise even more. They have all the pieces in place for many films to come in an ever expanding world if these rumors are true. The catalyst for the antagonists for each are in place now like dominoes it seems, and Days of Future Past is going to be the first one to fall that sets it all in motion.
Recent rumors surrounding the Fantastic Four reboot are suggesting it will use the title The Fantastic Four instead of the previous Fantastic Four: Reborn and that it'll be set after the events of The Wolverine. Could Reborn have been dropped in favor of using it as sequel title if Onslaught might be on the slate for a future film that may combine the teams? It does fit with other commentary suggesting Fantastic 4 cameos and crossovers with the X-Men film franchise and is one of the major stories with the two teams. Potentially even setting up possibilities for Misadventures in Babysitting type future crossover scenarios in solo character spin-off feature sequels much later on down the road.