TBH I think this qualifies as hijacking the prompt. You have no idea whether the prompter has any interest in reading about Sherlock as a trans character, and if you were going to reimagine the character in such an important way then you should have flagged your intention up front rather than at the end of yoru post, when readers might already be invested. It's really obnoxious to turn the prompter's wish into your own issuefic.