Disclaimer: Don't own HP, Avengers or OP.
Bruce is Fem!Harry:
Bruce Banner is in reality Elizabeth Potter, a female disguised as a male. She was somehow transported at a young to the world of One Piece and was captured by the Tenryubito. (How she got to the OP world and back is your choice.)
She had a daughter, who she hates (wrong I know… but she's a Tenryubito through and through).
The Hulk is still male and protective of Bruce (Bruce is more accepting of Hulk). In a way the Hulk is a physical manifestation of Bruce's magic, which she is incapable of using anymore (could be the trauma or the radiation).
Not even Fury knows Bruce isn't really 'Bruce'.
Only Betty knows so far, Bruce having begged her not to tell anyone.
The other Avengers find out when Amora gives them a 'document' of Bruce's life and they are compelled to watch it (Bruce was better at resisting, though).