Perhaps an older, forgotten enemy of the Lost Boys could resurface. Maybe someone from their past in Neverland who they thought was defeated but has now returned with a vengeance. This could be a character who was once a Lost Boy himself but turned bad due to some dark influence in Neverland.
It could also be a new character, perhaps a dark sorcerer who is jealous of the magic in Neverland. This sorcerer might target Emma because she has an unknown power within her that could disrupt his own plans for the magical world. He would try to separate her from Peter Pan and use her power for his own evil deeds.
Captain Hook would be an obvious choice. His hatred for Peter Pan could extend to Moira when she becomes Peter's friend. He might see her as another obstacle in his quest to get revenge on Peter.
The mermaids could be the main antagonists. They are known to be jealous and territorial in the Peter Pan stories. If the lost girl catches Peter Pan's attention in a way they don't like, they could make her life in Neverland miserable. They might use their alluring voices and their knowledge of the waters around Neverland to try and drive her away or lead her into danger.
It could be a new character. Maybe a dark sorcerer who has come to Neverland while Peter was growing up. This sorcerer wants to take over Neverland and sees Peter, with his new adult understanding and power, as the only obstacle. The sorcerer could start to corrupt the island, turning the mermaids and Lost Boys against Peter, making him fight not only for his place in Neverland but also for the very soul of the island.
Perhaps a new student named Tiger Lily. She comes to the high school and has her own set of rules and ideas. She wants to take over Peter's group of friends and tries to drive a wedge between Peter and Wendy. She's very competitive and sees Wendy as an obstacle to getting closer to Peter.
Peter Pan himself is often very popular. He's the leader of the Lost Boys and his charm and adventurous spirit make him a central figure in many fanfictions.
Captain Hook would be a likely main antagonist. He has a long - standing feud with Peter Pan, so he would see Peter Pan's daughter as a target or a way to get revenge on Peter. He might try to capture her or use her to lure Peter Pan out of hiding if he's still alive in the fanfiction's version.
One idea could be exploring the backstories of the Lost Boys in more detail. Maybe one of them has a hidden talent or a past connection to Neverland that even Peter doesn't know about.
One interesting story could be about a modern - day girl who stumbles into Neverland and has a romantic encounter with one of the Lost Boys. It might explore their different worlds colliding.
Unfortunately, I can't tell you the author of this particular fanfiction. Fanfiction can be written by anyone, from amateur writers to more experienced ones. To find out the author, you would probably need to search for it on popular fanfiction websites, or if it has any specific markings or credits within the text itself.