I wonder what kind of 粉丝 Fiction to write...

Well, if you're a new 粉丝 fiction author, I'd suggest picking the 来源 material you're most familiar with, and 写作 about it in canon style. It's simpler than alternate character interpretations, OCs, AUs, and anything else deviating (radically 或者 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.