It’s not the simple fact of her presence that’s the problem. It’s the nature of her possible dynamic.

Tony Stark was born super-wealthy to a super-intelligent father, he had every advantage and spent a good portion of his life being a cookiecutter self-centred wealthy playboy. It took all of his resources and some fairly traumatic experiences for him to become Iron Man, and he has all of that history lending some depth to his character. This new character has none of that. She came from the streets, went to MIT on a scholarship. There’s not going to be a process to her becoming Iron Man, she’s simply going to be Iron Man, with none of the privilege that allowed Tony to become Iron Man. From the get-go she’s already placed on a higher platform than Tony because any demons she has are going to be easily justifiable by her disadvantaged background. It’s not a recipe for an interesting character. The only thing that kept Iron Man balanced was that while he was super-intelligent and super-powerful he was ultimately selfish and supremely flawed. She’s not going to have any of that.

I have a similar problem with making Amadeus Cho the Hulk.