As someone who has defended a number of decisions Singer made with Apocalypse...
Just no, no, no, no, no. With each film, Lawrence becomes more apathetic. I liked her in FC, but she was asleep in Apocalypse. So, if she did it, it would only be for the money. Additionally, no fan really wants to see it, so its box office will probably be quite bad. And finally, I do not think this character can support her own film. There are female X-Men characters like Storm, Rogue, maybe Jean (but her biggest stories all involve the team reacting to her) who could... but not Mystique.
So overall, just nope.