If Gambit's in Apocalypse, he'll be almost 100% guaranteed that he'll be in a GOOD movie, and it'll be a great way to promote the solo film. Maybe a Quicksilver-size role in DOFP would be ideal, but somehow I'd love to see him in the present, so maybe adjust his powers where he ages slowly, there's about a dozen mutants out there that have this ability.
I want him in X-Men team films, because that way he'll have a solid future in the film series. If the solo film fails, he'll get that second chance to redeem himself in team films.