Kit Harington's Black Knight from Black Widow's life after Captain America: Civil War as she is reunited with her Russian family and faces off against Taskmaster.
Although Taskmaster is a new villain for the MCU, the trailers for Black Widow indicate the character is Black Panther with his claws, and Hawkeye with his bow and arrow. But, Taskmaster will also have a sword in the movie that could be tied to the MCU future instead of the past
There Ebony Blade and Black Knight. This same connection could be maintained in the movies and give all of Taskmaster's weapons links to other characters in the MCU, with the specifics of such a tie able to play out in multiple ways.
If Taskmaster is linked to Black Knight, there are two different paths that the MCU could take to make this possible. Although Dane Whitman is the version of Black Knight who will be in Eternals, he was not the first to hold that mantle or the Ebony Blade in the comics. His ancestors had the sword for generations so it is possible that Taskmaster was able to study a previous version of Black Knight. The comics also with Black Widow in one of the trailers.
In either scenario, Black Widow could then have substantial implications for Black Knight and Eternals. The film could reveal a legacy for Black Knight that Dane now has to follow in, which would help shape his arc in Eternals and beyond, or make him a veteran superhero. The possibility of the latter would mean there could be multiple years of Dane suiting up before meeting the Eternals, while the former also opens the door for other versions of Black Knight to appear in the MCU's past. In fact, that could even be shown further since Eternals will span thousands of years. And in addition to being great pieces of world-building, this small connection would be just one example of how Black Widow is shaping Marvel's Phase 4.