Gambit could easily work with the OT cast film. They did bring Nightcrawler in X2 and Beast in X3 but just because Rogue is in the picture, it doesn't mean they have to do the Rogue/Gambit pairing.
Truth is they just screwed up that storyline by having Rogue in the OT cast and then having Gambit in Origins. Oh well.
In hindsight, the Origins film didn't any good to the series. They could have just moved forward with a X-Men 4 movie and introduced Gambit there.
"This is our chance to make a difference" - Sue Storm - The Invisible Woman
"He's stronger than any of us but he's not stronger than all of us" - Reed Richards - Mr. Fantastic
"We could these powers to help people" - Johnny Storm - The Human Torch
"You can't fix this, nobody can" - Ben Grimm - The Thing