The most recent Marvel movie by Fox to be revealed as in development is Kitty Pryde, by none other than Deadpool director Tim Miller. No information is known about who is set to play the titular character or when it's set to go into production (there's nothing guaranteeing it'll even make it that far), but it's reportedly being actively worked on by Miller and co.
Ellen Page previously played the phasing mutant in the X-Men universe, and the character has been a staple in the comics since her inception. Studios will be looking for the next big female superhero after the resounding success of Warner Bros.' Wonder Woman in 2017, so it wouldn't be a surprise for Fox to fast-track this for early 2019, or sooner.
Recent Golden Globe winner James Franco has teamed up with his brother, Dave Franco, to produce an R-Rated movie following Multiple Man, set within the X-Men universe. Wonder Woman scribe Allan Heinberg has worked on the movie with Franco, and X-Men: Dark Phoenix director Simon Kinberg is in line to produce.
Ramona Films, a company owned by the Franco brothers and Vincent Jolivette, is co-producing the flick alongside Kinberg's Genre Films for Fox, though there's no word on when the movie is expected see the light of day. It's also possible that the recent allegations of sexual misconduct made against Franco could derail the potential film completely.
Jamie Madrox, otherwise known as Multiple Man, has been portrayed in live-action once before, with Eric Dane appearing in X-Men: The Last Stand.
One of the brains behind FX's acclaimed Legion, Noah Hawley is developing a movie based on one of the most popular villains in Marvel Comics history: Doctor Doom. After being teased at last year's Comic-Con International in San Diego, it was recently confirmed that Hawley is still developing the film despite the impending acquisition. He'll be working on Doctor Doom once he has finished the script for Season 2 of Legion, though not much else is known at the moment.
Doom has appeared in live-action a few times, most recently played by Toby Kebbell in 2015's critically-panned Fantastic Four reboot. Mad Mikkelsen has expressed his interest in the role. Dream casting, yes, but a casting that would be difficult if the movie ends up taking place in the MCU, since the actor played Kaecilius in Doctor Strange.