A group of rogue demigods could be the main antagonists. They believe that Percy is too powerful and his presence is causing more problems than solutions in the demigod world. So they make him disappear. But when Percy comes back, he shows them that they were wrong and that they have been mislead by false beliefs.
Well, he might disappear because of a powerful curse from an ancient enemy. Maybe he gets trapped in a different dimension. When he returns, he could have new powers or knowledge. For example, he might have learned some secret magic from the other dimension that helps him defeat a new threat in the Percy Jackson world.
In the 'Percy Jackson Disappears' fanfiction, perhaps Percy could be kidnapped by an unknown enemy. Maybe it's a new villain who has a vendetta against the demigods. This would lead to his friends, like Annabeth, going on a desperate search for him, facing many dangerous challenges along the way.
The gods are always a possibility. Zeus, for example, has been distrustful of Percy in the past. If Zeus feels that Percy is getting too powerful or is a threat to his rule, he could engineer a situation where Percy is betrayed. He might use other demigods or even mortals to turn against Percy.
One possible plot could be that Percy is betrayed by one of his closest friends in Camp Half - Blood. Maybe Annabeth turns on him out of jealousy over his growing powers or a misunderstanding. Percy, feeling deeply hurt, decides to disappear into the mortal world. He cuts off all ties with the demigod community and tries to live a normal life, but his past keeps haunting him as monsters still sense his presence.
The setting in which Harry disappears and returns also matters. If it's in the middle of Hogwarts, the school would be in chaos. Teachers would be trying to keep the students calm while secretly searching for Harry. And when he returns, the reactions of different houses - Gryffindor being relieved and proud, Slytherin perhaps being suspicious - could add more layers to the story.
The theme of sacrifice could also be present. Maybe Harry disappeared because he sacrificed himself for a greater good. And his return could be a sort of resurrection, where he has to deal with the consequences of his sacrifice and how it has affected the wizarding world.
In the 'Percy Jackson returns to Goode' fanfiction, it could be that Percy has to face some challenges related to his dual identity. On one hand, he is a normal student at Goode, on the other, he is a demigod. This could lead to some interesting situations where he has to balance between the two worlds. For example, he might have to hide his demigod skills during a school competition but still manage to win in a rather unexpected way. Also, there could be new characters introduced, perhaps a new demigod who also attends Goode and Percy has to team up with them to face a common threat.
Well, he might face new monsters. For example, some ancient and more powerful versions of the ones he's already fought. Maybe there are new quests waiting for him at the camp, like finding a lost artifact that has the power to either save or destroy the camp.
Obviously, Ash would be the main character. Then there could be female characters who form his harem. Maybe some of his long - time friends like Misty and Brock could also be involved in some way, even if they are not part of the harem directly. There might also be new characters that he meets during his disappearance.