Gambit is more likely to be used In
1:Sequel to DOFP with remaining FC cast
2;Sequel to DOFP with OT cast
I think 2 Is more likely so they can pair rogue and Gambit together.They will likely fudge the age and have gambit's appearance In origins be one of things changed because of time travel In DOFP.
Best comic book films-Superman X2 The Dark Knight X-Men Days of Future Past(rogue Cut) Batman Returns The Amazing Spider-man,X-Men,Captain AMerica:The Winter Soldier,Batman(1989) X-Men:Apocalypse