While "Black Widow" was notable for finally giving Scarlett Johansson's Natasha Romanoff a proper solo movie, many critics argued that Florence Pugh stole the film with her portrayal of Natasha's sister, Yelena Belova. That said, it's still fun to speculate about what could have been when it comes to casting.
According to a 2019 report by That Hashtag Show, Emma Watson was originally the frontrunner for the role of Yelena Belova. The same report notes that actors Alice Englert, Dar Zuzovsky, and Pugh herself were all being considered for the role at the time.
It's unclear exactly why Watson didn't play the "Black Widow" character, especially considering her established star power. Of course, Tinseltown is a chaotic place where parts and offers are constantly shifting, so it could have been for any number of different reasons.