Perhaps a new, powerful demigod - vampire hybrid that Percy has to face. This hybrid could be causing trouble in Forks, and Percy has to stop it before it destroys the delicate balance between the vampires, werewolves and humans there.
In a fanfiction where Percy Jackson goes to Forks, he might be really confused by the gloomy weather. He's used to more sunny and open places. He could meet the Cullens and there would be a clash of his demigod powers and their vampire abilities. Maybe he'd mistake them for some new kind of monster at first.
One idea could be Percy getting involved in the vampire - human drama in Forks. Maybe he discovers the secret of the Cullens and tries to find a way to co - exist with them, given his own experiences with different species in his own world.
One possibility could be that Hermione uses her magic skills in Forks in unexpected ways. Maybe she helps the local people solve some strange problems that they can't figure out with normal means. For example, she could use a spell to find a lost item in the forest which is full of mystery in Forks.
She could meet Bella Swan. They might start off as a bit of an odd couple, with Bella's more reserved nature compared to Hermione's boldness. But they could learn a lot from each other.
Well, perhaps Harry could be a wizard - creature hybrid. In Forks, he might discover that the vampires there have a connection to the magical world he comes from. Maybe he uses his magic to help the Cullens with some sort of threat that normal vampire powers can't handle.
One interesting plot could be Stiles forming an unlikely friendship with the Cullens. He might bring his unique sense of humor and problem - solving skills to Forks. Another plot could involve Stiles getting caught up in the vampire - werewolf drama there, perhaps using his knowledge from his own supernatural experiences to try and keep the peace. Or he could be on a mission to find something in Forks that is related to his own world's mysteries.
In the fanfiction, if Harry Vampire goes to Forks, he could potentially fall in love with a human there, just like Edward did in the original Twilight story. But his vampire nature might make things more complicated. He could have to struggle between his love for the human and his vampire instincts. Also, he might face challenges from the local vampire community if they don't accept an outsider like him.
Obviously, Percy would be the main character. There could also be some characters from the original Percy - related universe who might make an appearance to help him adjust or just to check on him. And from the Forks side, characters like Bella or Edward (if it's in the Twilight - Forks setting) could potentially cross paths with Percy. Maybe they become friends or foes depending on how the story unfolds.
Well, it could be about Percy adjusting to the new environment in Forks. Maybe he has to deal with the gloomy weather and the small - town vibe that's so different from what he's used to. He might meet new friends or face some challenges specific to Forks.