While doing press for her new movie Passengers, Jennifer Lawrence revealed that she's willing to suit up as Mystique again for her Passengers co-star Chris Pratt in a Guardians of the Galaxy cameo. But she's not willing to undergo the makeup for another X-Men film.
"I would choose that over doing another X-Men movie maybe. I won't be Mystique in the X-Men movies, but I would love to be Mystique in the Guardians of the Galaxy. I wonder if that can happen."
It can't. At least according to Kevin Feige, who recently said, "It's an impossibility at this juncture," in regards to the X-Men ever coming to the MCU.