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 | JEAN GREY | PORTRAYED BY FAMKE JANNSEN
"All kind of things." - X1
"This is the only way" - X2
"Kill me before I kill someone else" - X3
"Where else would I be?" - X4