It remains to be seen if those latter two projects will ever actually be made, but since Fox is joining the Disney family soon, we can reasonably assume that the Mouse House doesn’t want to interfere with Dark Phoenix and The New Mutants’ performances, hence why development on any reboots won’t happen right at the beginning of next year.
Dark Phoenix and The New Mutants are scheduled for release on June 7 and August 2, 2019, respectively, and X-Force and Gambit are also still in development.
In the meantime, there are still a few X-Men movies left to close out the Fox era.