So it has already been acknowledged that inserting Deadpool into the main X-men films, or even the other possible/planned spinoffs would be hard considering he is circa 2016 and the X-films will only hit the 80's with the next one; however, Cable's entry opens up the door for messing with the chronology and using characters in different eras than where they should technically fit. This leaves the potential problem of contradictions, yet Deadpool establishes that he and the X-men have crossed paths before so it would not even contradict what they have put out to have him encounter them so long as he does not become a team member (which should not happen irregardless). The reverse is also true were they to want to have an X-men character from the earlier timeline crossover into future (pun intended) Deadpool movies. Naturally he would need to get handled a little differently than in his solo outing, but it wouldn't even need to take up much time to throw him in. Potentially it could get done with one or two throw away lines (like the ones they forgot to put into DoFP to tell the more casual viewers how Charles came back from the grave).