Well, one possibility is that he comes back to help his old friends deal with a new supernatural threat. Maybe there's a powerful and mysterious creature that the remaining pack members can't handle on their own, and Stiles' return is the key to their survival.
Perhaps Stiles returns to Beacon Hills because he has a premonition or some sort of sign that his friends are in danger. He's always been the brains of the group, so his return could bring new strategies and ideas to fight whatever is plaguing the town. He might also have new powers or knowledge that he acquired during his time away, which could be crucial in the battle against the unknown.
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.
Maybe he meets Lydia. She was always an important part of the group, and her intelligence and supernatural abilities might lead her to be the first to cross paths with Stiles. Their encounter could be filled with a bit of mystery and a lot of relief that he's back.
They're compelling because they offer new perspectives on the characters and the town. It's interesting to see how things change without Stiles.
The town of Beacon Hills itself might be affected. Stiles was always in the middle of things, helping to solve supernatural mysteries. Without him, there could be an increase in unexplained events. And some of the townspeople who knew Stiles, like Sheriff Stilinski, would be deeply affected on a personal level, with the sheriff possibly becoming more overprotective of the remaining youth in town.
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.
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.
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.
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.
Well, maybe she'd use her magic skills to solve the supernatural mysteries in Beacon Hills. For example, she could help the pack with her spells when they face a really tough enemy that normal means can't defeat.
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.