Deadline has reported that 20th Century Fox has hired Brian Michael Bendis to write a new X-Men movie to be directed by Deadpool directed Tim Miller. At the moment, all we can confirm is that it is set in the X-Men universe, though who knows what timeline, and that it is known as '143'.
Bendis recently made the jump from Marvel to DC as a comic book writer but it's not surprising that Fox would try to bring into the fold in some capacity — particularly since Fox/Disney doesn't seem to be a done deal yet. Bendis helped shape the Marvel universe in massive ways, including creating Jessica Jones and Miles Morales, the latter due to make his big screen appearance in the winter. X-Men is a Fox property so while he's going back to Marvel he's not really going back to Marvel. Miller is currently in pre-production on the latest Terminator feature.
As for what the 143 stands for no one at Fox is talking yet. X-Men issue 143 is directly after the Days of Future Past story. It's a world where the Phoenix no longer exists, which would make sense considering one of the next movies is X-Men: Dark, and that story usually ends with Jean dying. There is a chance Fox is really happy with what they've seen so far of New Mutants and want a story that can emulate it. #143 is about Kitty Pryde trapped in the X Mansion with a demon. Horror movies are notoriously cheap, quick to make, and there have been rumors about a Kitty Pryde movie for years.
This all speculation though. What do you think 143 stands for?
UPDATE: The Hollywood Reporter claims to have sources that confirm it is a Kitty Pryde movie.
143 is a code name for the project which sources say will focus on the character Kitty Pryde, a mutant who can phase through objects. The character was played by Ellen Page in 2006's X-Men: The Last Stand and 2014's X-Men: Days of Future Past.
The project's codename is a reference to Uncanny X-Men No. 143, one of the earliest Pryde stories and her first solo adventure featuring her facing a demon alone in the X-Man.