Fox decided to do Days of Future Past right after X-Men: First Class.
Director Matthew Vaughn has revealed that the X-Men franchise as we know it could have been quite a bit different had he continued on after X-Men: First Class.
“My plan was ‘First Class,’ then the second film was new young Wolverine in the 70’s to continue those characters, my version of the X-Men.
Now, we know what he would have done differently, and it would have included a young Wolverine.