Eh, I've had ideas in my head similar to this, percolating around.

I've done retelling of Cinderella through trans lensing as a bedtime story, and have kinda sketched it out for eventual writing down in detail, but I've got three projects ahead of it that are going really slow because that's how I write: slooooooow.

This specific idea is a good one, but I think it would work best as a short story rather than a longer form. Maybe novella length at most. I can kinda see the layout of the story, and without a very clear vision of what happens after they've stepped into their prophecized role, it would fall apart past that point.

So it would be best served as a story with an open ended finish rather than a conclusion. That's not something that satisfies most readers. They don't think "and Rebecca walked out the castle gate, sword in hand" is an ending. But that's what the natural ending of this idea is. Once the girl is her full self, it's just another fantasy trope unless you're really careful, and even then wouldn't sustain very well without some serious passion on the part of the author.