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 | ICEMAN | PORTRAYED BY SHAWN ASHMORE
"Welcome to mutant high" - X1
"Call me Iceman" - X2
"Don't ever do that again" - X3
"They'll find us they'll always do" - X4
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