Re: Why not do both? X4/First Class... Combine The Franchises!
I think McAvoy/Fassbender/January/Jennifer/Caleb/Hoult/Lucas to join Hugh/Halle/Anna/Shawn/Colossus/Angel would be better in X-Men 5. Time travel is always nice and interesting.
Just give the original cast a solo film for themselves and the cast of XFC could join them in X5.