Never specified. All we know is Tim Miller was to direct it & the working title was "143." Rumored to be centered on Kitty Pryde, inspired by UXM #143, that classic Christmas Brood issue.
Lauren Shuler Donner On ‘Gambit’, ‘New Mutants’ Hulu Rumors & Future Of ‘X-Men’ Franchise Under Disney – TCA Nothing really new here. Just longtime X-Men producer (since the first film) Lauren Shuler Donner further confirming what we know: Dark Phoenix & New Mutants are as of now still hitting theaters (not Hulu or whatever) in 2019, and anything beyond that (including X-Force and Gambit) will be 100% up to Kevin Feige and Disney, not the old regime. If Donner is this open about the future, as opposed to Simon Kinberg's ambiguity, I wonder how Bendis and anyone else still toying with a script is approaching their work in light of that.