A popular fan-fiction trope that you strongly dislike
An exceedingly popular theme in Harry Potter fan-fiction is to have male characters born female instead, most commonly Harry, Ron and Blaise. I theorise that authors do this because they want to pair certain characters, but don’t want to include a same-sex relationship.
Whatever the reason, I strongly dislike fanfics featuring fem!Harry or fem!Ron (or fem!Draco, for that matter). I don’t mind stories where they are temporarily turned female due to a spell or a potions accident or something, and I enjoy stories where the characters transition. However, I don’t like stories where they are born female. I’m not sure whether Blaise’s gender is actually mentioned in the books, but I have a strong preference for male!Blaise.
Honestly, though, if you want to explore relationships between certain characters, just do it. It’s 2018 — there’s no need to switch their gender.