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.
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.
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.
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.
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.
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 possible plot could be that Stiles discovers he has a baby unexpectedly. Maybe an old fling left a baby on his doorstep. He then has to learn how to be a father while still dealing with his normal life, like school and his friends. He might struggle at first with things like changing diapers and getting up in the middle of the night, but with the support of his friends, he slowly gets the hang of it.
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.