My whole theory on parenting against the grain is to try and create space for my children, for all children, to make choices rather than being pressured to conform to defaults. This feels like winning.
The key here is that we didn't say, "be a superhero," but say, "You can be anything you want to be ... including a superhero." She can read and saw the word princess, but said, "I want to be batgirl!" And so it was.