According to our friends over at Collider, Deadpool director Tim Miller is currently developing a Kitty Pryde movie for 20th Century Fox.
Kitty Pryde, as you almost certainly know, is the young, female mutant with the ability to walk through (or "phase" through) walls. She can often be seen palling around with the X-Men, and has appeared in most of the X-Men movies (where she was twice played by Ellen Page). This is the first we're hearing of a solo movie.
Collider's careful to point out that "in development" does not mean "in production", and that Miller is also going to be tied up for the foreseeable future as he produces the Sonic The Hedgehog movie (shrug emoji) and directs the Terminator reboot being planned by James Cameron (slightly smaller shrug emoji). There's also that Disney/20th Century Fox merger to consider (it's remains unclear how that merger might impact production on the various X-Men spin-offs currently in development). In other words: if it's happening, it'll probably be a while before it arrives.
Obviously, there's no word on what Miller's vision might be for a Kitty Pryde movie, nor is there any indication who might play the titular role. For now, all we know is that a Kitty Pryde movie is in development and that - if Collider's to be taken at their word (and they're generally on top of this shit) - that Tim Miller's making it happen. Stay tuned for further updates as they roll in.