I wonder what kind of ファン Fiction to write...

Well, if you're a new ファン fiction author, I'd suggest picking the 情報源 material you're most familiar with, and 書く about it in canon style. It's simpler than alternate character interpretations, OCs, AUs, and anything else deviating (radically または otherwise) from the original source. It's also likely to garner less hate, even if it isn't incredibly original. Just don't rip off an existing episode.