Marvel Studios has brought Wonder Woman 1984 writer Dave Callaham on board to work on a Phase 4 project. At this time, it's not clear which movie that Callaham is working on within the Marvel Cinematic Universe. While the focus is currently on Captain Marvel, Avengers 4, and Spider-Man: Far From Home, the studio is also looking towards the future in their trademark secretive style. The upcoming Phase 4 is basically one big question mark except for the Spider-Man sequel, which kicks off the new Phase.
In addition to Wonder Woman 1984, David Callaham has also worked on The Expendables, Doom, and Godzilla. He's also currently working on Zombieland 2 and Mortal Kombat. As far as what he can be working on for the MCU, that is a mystery at this point in time. Though, he could be working on tweaking the script for Doctor Strange 2, which is getting ready to start filming. It's believed that Scott Derrickson and C. Robert Cargill have returned to write the sequel, but that has yet to be confirmed.
David Callaham could also be working on an upcoming Dark Avengers film. It has been rumored that Marvel Studios commissioned a script for the project, and Callaham could be the one coming in to either write from scratch, or take an existing script to refine it. It's also possible that he could be taking James Gunn's Guardians of the Galaxy 3 script to update it for all of the changes that will be occurring in Phase 4 before the film begins production, though that sounds unlikely. Regardless, whatever the screenwriter is working on will be a surprise to MCU devotees.
The standalone Black Widow movie is another project that the MCU has on the way, but the movie was written by Jac Schaffer. Cate Shortland has been tapped to direct the film, but it is possible that David Callaham could come on board to help out. As far as The Eternals is concerned, that project is currently being written by Matthew and Ryan Firpo, so that movie seems out of the question. With that being said, it's entirely possible that Callaham is working on a project that the public isn't aware of at this time.
Avengers 4 is going to end Phase 3 of the MCU, while Spider-Man: Far From Home will be the first Phase 4 film. It was believed that James Gunn's Guardians of the Galaxy 3 was going to help kick off Phase 4, but the movie is currently on indefinite hold since the director was fired by Disney over the summer. For now, it looks like Phase 4 will continue to be a mystery until Marvel Studios unveils the rest of their slate, which will more than likely be announced next summer at San Diego Comic-Con. This Marvel Studios news was first reported by That Hashtag Show.