1. Completely canon compliant - basically just the character's thoughts in a canon scene, or maybe expanding on a scene which is just mentioned in canon.
2. Character backgrounds/missing scenes/post-ending - stuff which is technically fanon but could fit into canon - your own take on a character's background, or a missing/omitted scene or your theory of what could happen to a surviving character after the end.
3. Tangents and what-ifs - stories inspired by ideas like "what if this character didn't die?", "what if my favourite pairing stayed together?" or "what if this character made a different decision at this point?".
4. Set in a different time/location but keeping the main theme - something like BTVS set in the 20s but still with Buffy being the Slayer, or OUAT in a non-Curse Enchanted Forest setting.
5. Completely AU - modern AU, human AU, high school AU sort of thing. Basically changing everything except the character names, appearances and personality traits.