I wrote a general post about what “fiction” was, but I never got into another category, writing fanfiction. Then again, I had yet to post about what a fandom was, for those people who may or may not know the correct terminology.
Fanfiction, essentially, should be exactly how it sounds. You are writing fiction based on what you are a fan of. For me, I prefer writing fanfiction about animes or mangas that I have seen. For another person, they may not like animes or mangas, but they have a favorite TV show. Or, if there is a book that you liked to a certain degree, but disliked a few minor points. By all means, feel free to write what makes you the most comfortable!