While she didn’t reveal which role, it stands to reason that Brie Larson might have been auditioning for Natasha Romanoff/Black Widow in Iron Man 2. Natasha is the only significant female character in that first sequel, and the search for the perfect actress was likely a wide one. Even if it’s hard to imagine anyone else besides Scarlet Johansson in the role.
Marvel fans can re-watch Brie Larson’s tenure in the MCU (plus the movies she didn’t get) over on Disney+. You can use this link to sign up for the streaming service.