And for the record, unless this is X-Factor, I don't see this happening. For once, I feel like there should be more X-Men team films and solo films distract from that when you consider that X-Force, Alpha Flight and X-Factor are often far more integral to the franchise than solo series and the team dynamics are the best part of the X-Men and what makes them special.
Why settle for just Mystique when you could have Mystique, Gambit, Rictor, Multiple Man, Siryn, Layla Miller, Forge and Polaris at the same time? Taking five or six of those nine for the roster and then greenlighting the project works. So does making Mystique and Gambit the stars of the show. It consolidates two proposed solo films into one which saves time and money and it also allows lesser known characters and actors to appear supporting them.
And when compared to Disney or Sony, Fox really does have the easiest time with regards to universe building because they do have these teams. Sony has all of one license while The Avengers are a collection of heroes who normally operate independently but come together to face down the big threats. Fox's strength is the ease of introducing characters since they're a collection of teams that all have a specific purpose that unites them.