“Vulture has learned that after finishing up next summer’s X-Men: Days of Future Past, Singer will indeed be behind the camera on the new film, making this the director’s first back-to-back foray into the X-Men world since he kicked off the franchise with X-Men (in 2000) and X2 (in 2003). But though Days of Future Past weaves together the veteran X-Men cast (including Patrick Stewart, Ian McKellen, and Halle Berry) and the younger actors introduced in X-Men: First Class (James McAvoy, Michael Fassbender, and Jennifer Lawrence), and Apocalypse will be a direct sequel set up in Days of Future Past’s closing moments, the new film is intended to focus strictly on the younger McAvoy-Fassbender cast.”
Vulture on X-Men: Apocalypse
You know what that means? If Apocalypse happens in the 70s, a thing that did NOT happen in the previous trilogy timeline - the future is once more open! X1-X3 are NOT predetermined, the way Charles and Erik’s relationship will develop is not be predetermined! Their future is once again a blank slate! How awesome is that?