They could easily include Gambit and Deadpool in a future X-Men team movie without recasting and retconning the Gambit/Deadpool that we saw in Origins. if this was the comic-books, this wouldn't be a big issue, in fact a lot of things like this happened in the comics.
X-MEN RI5E' MUTANT OF THE MONTH | COLOSSUS | PORTRAYED BY DANIEL CUDMORE
"I can help you" - X2
"Yeah, she took off" - X3
"Mystique" - X4