Mystique and bring the X-verse into modern times with it (this presumes that Fassebender and McAvoy are out after the next movie). Because she is a shape changer you can easily get away with still using Jennifer Lawrence. Just put in a throwaway line somewhere about how she uses her ability to look ever young and then never mention it again.
Now you can reboot the modern time X-men with the obvious recasting they'll have to do since all the originals are getting too old and a Mystique solo is the bridge to do it.
I would assume Mystique, JLaw is a hot commodity right now and she is the only one they bothered developing enough at this point.
Seriously Storm.
She's even ripe for merchandizing. Look at all the costume changes she's had over the years!
"Always listen to the crazy scientist with a weird van or armful of blueprints and diagrams." -- Vibranium
Storm. This can be loosely based on the Eric Jerome Dickey Storm graphic novel. For legal reasons, take out young T'Challa, and replace him with....