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 | SHADOWCAT | PORTRAYED BY ELLEN PAGE
"Who's hiding? ********" - X3
"Too late, ********" - X4