Tetsuya Nomura has basically won at fandom, hasn’t he? He wrote a fanfic so popular that in many circles it’s entirely eclipsed the source material. He even has people cosplaying as his wanky teenage OCs. If fandom has a victory condition, surely this is it!
(For those about to protest that Kingdom Hearts doesn’t qualify as fanfic: at one point, Mickey Mouse shows up wielding a sword and wearing a black leather trenchcoat, and starts spouting exposition about the fundamental duality of light and darkness. There is nothing more fanfic than that.)