The Nogitsune could also be a great antagonist. It thrives on chaos and mischief. Percy's presence and his powers would be something the Nogitsune would want to either corrupt or use for its own evil purposes. It could try to possess Percy or those around him to gain access to his demigod abilities.
Well, he might get involved in the supernatural chaos of Beacon Hills. Maybe he'd team up with the local werewolves or other supernatural beings to fight against a common enemy. He could use his demigod powers in unique ways, like using his water control against a fire - based threat in Beacon Hills.
One idea could be Percy arriving in Beacon Hills and being mistaken for a new supernatural threat. The local pack tries to figure out his powers and intentions. Another could be Percy teaming up with the pack to fight a common enemy that is drawn to the unique energy of Beacon Hills and Percy's own demigod powers. Maybe he also has to adjust to the high school life there while hiding his true identity from most of the students.
There's also a chance that he'd meet Lydia first. Lydia has her banshee powers, and she might be drawn to Percy because of his otherworldly presence. She could sense something about him that's not of this world, and approach him out of curiosity. Their meeting could be a very intense and mysterious one, with Lydia trying to figure out what exactly Percy is.
Well, Hermione is a strong - willed character. In Beacon Hills, this would serve her well. She would not be afraid to stand up to the powerful supernatural forces there. She could become a sort of mentor figure to the younger characters. She might teach them the value of knowledge and how to use it in the face of danger. Also, her magic could be a great asset in battles against the various threats that the town faces on a regular basis.
The plot could involve Hermione using her magical skills in the world of Beacon Hills. Maybe she helps the pack with some supernatural threats using spells and potions. For example, she could create a protection spell around the town to keep out certain dangerous creatures.
One possibility is that Harry's arrival in Beacon Hills shakes up the existing power dynamics. If there are different factions in Beacon Hills, like werewolves, hunters, etc., Harry could be an unexpected wildcard. He might form alliances with some while being a target for others. His magical abilities could either be a great asset or a cause for fear among the residents. For example, if there's a conflict between the werewolves and some dark force, Harry could use his magic to help the werewolves in ways they never expected, but this might also draw the attention of those who don't trust magic in the town.
One possible plotline could be Bella adjusting to the supernatural elements in Beacon Hills. Maybe she discovers the existence of werewolves and other creatures and has to learn to co - exist. Another could be her forming new friendships or even romances with the local characters.
One possible reason could be to protect his friends. Maybe he discovers a new threat that is specifically targeting him, and he decides to leave Beacon Hills to draw the danger away from those he loves. For example, in some fanfictions, a powerful and vengeful supernatural being might be after Stiles, and he believes that by leaving, he can keep his friends safe.
Well, perhaps Harry could bring his magic to Beacon Hills. He might use his spells to help the locals deal with the supernatural threats they face there. For example, he could use a Patronus to fend off some of the dark creatures that lurk in the town at night.