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 | NIGHTCRAWLER | PORTRAYED BY ALAN CUMMING
"Most people will never know anything beyond what they see with their own eyes?" - X2
NIGHTCRAWLER WILL RETURN IN X-MEN RI5E