During an appearance on Empire’s podcast, Singer said that Mystique would be “right for [a solo movie].” Jennifer Lawrence has suggested she might not return to the role following X-Men: Apocalypse, but that doesn’t seem to diminish this spinoff’s chances at all.
Singer said that Mystique "has this different view of the world. Xavier can get into Cerebro and look at the world but he’d rather just teach classes and see the beauty of mutants and humans co-existing in his mansion in Westchester. Along comes Raven with a reality check on the state of the world. It opens up a lot of avenues."
If this project goes ahead, Mystique would join Wolverine, Gambit, and Deadpool as X-Men characters to get spinoffs.