It appears as if Fox's X-Men is answering Marvel's The Avengers: Infinity War movies with a 2-part film of their own.
According to Olivia Munn, who played Psylocke in X-Men: Apocalypse, the upcoming Simon Kinberg-directed X-Men: Dark Phoenix movie will be a 2-part flick.
“I don’t know what I’m supposed to say,” Munn told Collider when asked whether she’s now working on X-Men: Dark Phoenix. “I’m supposed to be vague. That’s my answer. I’m horrible at that. It’s like a two-parter, this movie. That’s more than I’ve said, in general. I don’t know why people have to be so secretive.”
From Olivia's Munn's statement, it appears as if she and Psylocke will be a part of the second movie, whatever it's called.
An X-Men fan site is also claiming that an extra on set has stated after filming is completed on X-Men: Dark Phoenix, that the studio is having them back soon to start filming on the second movie.
X-Men: Dark Phoenix has a November 2, 2018 release written and directed by Simon Kinberg also starring James McAvoy as Chrales Xavier, Michael Fassbender as Magneto, Nicholas Hoult as Beast, Alexandra Shipp as Storm, Jennifer Lawrence as Mystique, Sophie Turner as Jean Grey, Tye Sheridan sas Cyclops and Kodi Smit-McPhee as Nightcrawler.