His best friend could discover it. Since they are so close, they might notice something different about Stiles that makes them suspect he's actually a girl.
Well, it could be that Stiles is secretly a girl and in the fanfiction, this secret is slowly revealed as he falls in love with someone. And the person he loves starts to notice little things that don't quite add up with his supposed male identity, like his more delicate features or his different style of communication.
Some of his friends might be really surprised. Scott, for example, might be a bit worried that Stiles' vampire obsession could lead him into dangerous situations. Lydia, on the other hand, could be more intrigued and might want to know more about the fanfiction. She might see it as an interesting intellectual pursuit.
One possible plot twist could be that Stiles has been a werewolf all along but has been using some sort of magic or potion to suppress his werewolf side. This could be revealed when he is put in a life - or - death situation and his true nature emerges to save the day.
One common theme is the struggle to maintain a normal life. Stiles often has to deal with hiding his fame from those closest to him, like his friends and family. Another theme is the allure of the fame itself. Sometimes he's tempted by the perks of being famous, but also realizes the downsides.
A plotline might be that Stiles got secretly married while on a trip away from his friends. When he returns, he has to figure out how to integrate his new married life into his old friendships. There could be comical moments like trying to hide his wedding ring or coming up with excuses for why he can't hang out at certain times. Also, the relationship within the marriage could be explored. Is it a marriage of convenience that slowly turns into love? Or was it a passionate love - at - first - sight kind of marriage that they now have to deal with in secret.
Well, it could be that Stiles' secret vampire fanfiction is actually a premonition. He starts writing about a particular vampire who then shows up in real life. But this vampire is different from the typical ones in the stories. Instead of being evil, it's trying to find a way to co - exist with humans. Stiles then has to use his knowledge from the fanfiction to help this vampire, all while still keeping his vampire - loving side a secret from his friends.
It could be Lydia. They have a long - standing relationship in the show, and a secret marriage between them would be quite a shock. Maybe they eloped in a moment of passion and are now trying to figure out how to break the news to their friends.
I'm not sure about a specific one, but you can start by looking on fanfiction.net. Just search for 'Stiles is secretly famous' in the search bar. There are likely many different takes on this concept there. Some might focus more on his friendships, while others could be more about a romantic aspect of his secret famous life.
A frequently seen theme is the reaction of the other characters. Some might be shocked, while others quickly adapt. Also, the theme of facing challenges differently as a girl comes up often. In some fanfictions, she might face more discrimination or different types of expectations.