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: DAYS OF FUTURE PAST
The future of X-Men's original trilogy cast begins this May!
Hugh Jackman - Halle Berry - Ian McKellen - Patrick Stewart
Anna Paquin - Shawn Ashmore - Daniel Cudmore - Ellen Page