Marvel's upcoming Black Widow movie poses a problem for the MCU's timeline. Marvel Studios used their Hall H panel at San Diego Comic-Con 2019 to reveal details of their upcoming Phase 4 slate. Next up is Black Widow, now confirmed to release on May 1, 2020.
The SDCC footage revealed a lot of big information about the movie. Black Widow's villain is Taskmaster, Florence Pugh is playing Natasha's successor Yelena Belova and David Harbour is Red Guardians. But, perhaps biggest of all, Kevin Feige also confirmed how Black Widow fits into the overall continuity of the MCU; it's set between the events of Captain America: Civil War and Avengers: Infinity War.
Of course, Marvel viewers already know a lot about what happened to Black Widow between those two films. In the aftermath of the superhero Civil War, Natasha became one of the world's most wanted, and she joined up with Steve Rogers, Falcon, and Scarlet Witch in a "Secret Avengers" team of illegal vigilantes. In order to remain hidden, Black Widow dyed her trademark red hair white; she helped the rest of her team work out how to hide more effectively as well. However, there's no evidence the rest of the Secret Avengers team are set to appear in Black Widow, and every image of the film has shown Natasha with red hair. That suggests a rather intriguing adjustment to the canon: where can Black Widow fit in the MCU timeline?
While the first two phases of the MCU were mostly told in chronological order, by Phase 3 Marvel began to hop around the timeline with impunity. Guardians of the Galaxy Vol. 2 was set back in 2014, Captain Marvel in 1995, and a number of movies have already explored the consequences of Captain America: Civil War years after the fact. The problem with this approach is that it can get increasingly difficult to keep the timeline straight when a movie released in 2022 is really set in 2016 and before a lot of other films; that invites continuity problems. It's quite surprising to see Marvel continue with this strategy at the very beginning of Phase 4.
Black Widow on a very public mission with red hair after Captain America: Civil War looks to invite that, although there is a possible solution. Natasha Romanoff was initially part of Team Stark in Captain America: Civil War, and switched sides during the Airport Battle. When Iron Man heard what she'd done, he resolved to let Black Widow go; she was never taken to the Raft with the rest of Steve Rogers' team, and thus was never broken out by Rogers. It's entirely possible Natasha didn't join up with Captain America straight away, but instead operated solo for a while. In fact, that would make sense; Steve Rogers had gone to ground, and it would presumably take some time to track him down.
So, assuming Marvel is careful Black Widow can fit before Infinity War and have Natasha still with red hair. It would also explain why none of the Secret Avengers seem set to cameo in this film (unless a Falcon surprise is being kept secret).