Marvel Studios' Phase Four line-up is (for the most part) a mystery, as President Kevin Feige has said the studio currently has no plans to announce anything until after Avengers 4 hits theaters next summer. However, a new report states production is slated to begin on one of the studio's upcoming film's next summer.
According to Geeks Worldwide, Marvel Studios is set to begin production on an unspecified Phase Four film June 2019 in the United Kingdom. While it remains unclear just which film this could be, the site speculates on three possibilities: Black Widow, Doctor Strange 2, and The Eternals.
Of the three options, Black Widow could be the most likely of the prospects. After all, not only has the film secured a director in Berlin Syndrome helmer Cate Shortland, but Geeks Worldwide reports casting is also rumored to be underway for the project. The film was officially greenlit earlier this year following the news Jac Schaeffer was brought onboard to pen the feature. Details surrounding a Black Widow film are sparse, though it's believed the film will be a prequel set before the events of 2012's The Avengers. With that in mind, moving production overseas to the United Kingdom for one of Black Widow's earlier adventures would make sense.
As for the other two options? Feige recently confirmed Doctor Strange 2 would happen, and director Scott Derrickson seemed to suggest something would be happening come September. However, it's unclear how far along pre-production work is on the sequel, and whether or not a script has been completed, making Doctor Strange 2 an unlikely option.
On the other hand, The Eternals brought on writing duo Matthew and Ryan Firpo back in May, with the expectation the project would be presented to directors once a completed script had been turned in. It's entirely possible the project will be going out to directors shortly, indicating pre-production may not be that far off.
The only other films confirmed for Phase Four are Spider-Man: Far From Home, which is currently filming, and Guardians of the Galaxy Vol. 3. Given that Marvel Studios has yet to begin its search for a new Guardians director to replace James Gunn, the film could be temporarily delayed.
Filming was expected to kick off in January, with Gunn having recently turned in a completed first draft. Following his firing, it's now reported Marvel Studios wants to use his draft, with an incoming director given the chance to tweak the script to their liking. Should the studio secure a director relatively soon, it's more than likely that, even if production should be pushed, filming will take place long before June of next year.
As of right now, while Marvel Studios has announced a slew of release dates for movies beyond Avengers 4, none of the announced dates have been publicly assigned, although both Guardians of the Galaxy Vol. 3 and Black Widow are thought to be set for 2020 releases.