Doctor Strange turned the MCU on its head when it was released in 2016. While the cast of that film was very strong, it could have been even better. It could have had Jessica Chastain. In an episode of Junkfood Cinema, the film's screenwriter C. Robert Cargill revealed that the actress was approached by director Scott Derrickson to have an important role in the film:
This is the interesting thing that Hollywood's starting to find out, is that a lot of the actresses out there want to be superheroes as much as the actors do. I know this happened with Scott [Derrickson]. He went to Jessica Chastain to get her on Doctor Strange, because we were considering her.
And she's like, 'Hey, look, this project sounds awesome, and I would love to do it. But I'm only going to get one shot at being in a Marvel film and becoming a Marvel character; and I trained in ballet, and I really want to wear a cape.' And that was the coolest rejection ever. She wanted to be in a Marvel movie, but she wanted to be the superhero, not the badass doctor Night Nurse.
As a fan of her's that would have been awesome. She has appeared in a superhero film since, last year's Dark Phoenix. Too bad she didn't hold out for something better. Maybe Doctor Strange in the Multiverse of Madness has a role for her?