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.
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.
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.
One idea could be that Stiles is turned into a vampire against his will. Maybe he's attacked one night while out investigating something supernatural. As a vampire, he has to struggle with his new urges, like the thirst for blood, while still trying to be the same person his friends know. He might start to see the world in a different light, both literally and metaphorically, as his vampire senses heighten.
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.
The themes usually include the conflict between the human and vampire sides of Stiles. He may still have his human emotions and values, but the vampire instincts can be a challenge. Love and loyalty are also big themes. His friends may or may not accept him as a vampire, and he has to decide how far he'll go to protect them. And there's often a theme of mystery. The vampire world has its own secrets and rules that Stiles has to figure out.
One common theme is the struggle with identity. Stiles has to come to terms with being a vampire instead of human. Another theme is the relationship with his friends. His friends might react differently to his new state, and that's often explored. Also, the control of vampire powers, like the thirst for blood, is a common theme.
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.
His relationship with his friends would likely become more strained. They might be afraid of him or he might be afraid of hurting them. For example, his best friend Scott, who is also involved in the supernatural world, might be worried about Stiles losing his humanity.