Well, I never ever outlined a damn thing for fanfiction. Not long fics, not short fics, not anything. I think you have more leeway with fanfiction, quite honestly. Your audience already understands your characters, no matter how drastically you want to alter them. They already know the setting, and you only need to nudge a tiny bit to spin off in a new direction.

I didn't start outlining anything until I stopped writing fanfiction.

And I'm not recommending any kind of rules for writing. I hardly know what I'm doing myself, let alone presuming to dictate anything like rules.

All I can say, in my honest opinion, is that without an outline, all my original fiction was too distant, too navel-gazing, and too confused. I can't speak to anyone else's writing.