Welcome to Mibba! I'm Hina. I agree with Nicole, try not categorizing your writing right away, especially if you're not sure oft the category. If you're focusing on a certain fandom then try thinking of stuff that hasn't been done to them and how they would react to certain situations.
Hello, and welcome to Mibba! If I were you, I wouldn't try to categorise your writing before you begin/as you're writing. Just write what comes to mind, then see if it's AU and what genre afterwards, otherwise you might not enjoy writing your story if you're too busy worrying about its genre! I always do those sorts of things afterwards, when it's easy to tell