Don't forget Wolverine 3. From 2016 to 2018, it could be the Apocalypse trilogy. With Apocalypse serving as behind-the-scene type of villain in Wolverine 3.
I think this will be their "using the original trilogy cast" as well as confirm what Lauren Schuller Donner has said was going to be a "true X4." Not to mention, there is a "mystery" X-men film on the docket.
Actually Shuler Donner brought up Days of Future Past when she was asked if we were gonna see the original cast again, then she said X4 leads to X5. Who knows if DOFP was her idea to X5 that she presented to Fox.