Well, one possibility is that the other characters would be in a state of panic. Grover might use his nature - related powers to try and track her down. The demigods would likely form search parties and head to all the places Annabeth was known to frequent, like the Camp Half - Blood or the various locations in the mortal world where she had quests. They would also probably start to suspect some of the old enemies, like the Titans or the Giants, of being behind her disappearance.
The story would be full of twists and turns. Percy would be at the forefront of the search for Annabeth. He might face a lot of challenges along the way. For example, he could be tricked by some magical illusions set up by an unknown enemy. The other characters would also be involved in different ways. Nico could use his powers over the Underworld to see if there were any signs of Annabeth there. Meanwhile, the gods might also take an interest, especially Athena, as Annabeth was her daughter. There could be a race against time to find her before something bad happened to her, and Percy would stop at nothing to get her back.
If Annabeth goes missing instead of Percy, the story could take a very different turn. Percy would be desperate to find her. He'd probably start by retracing her steps, talking to all the people she had interacted with recently. Maybe he'd discover some clues she left behind that were related to her architectural interests, like some ancient building plans or a strange symbol she had been studying. The other demigods would also be on high alert, fearing that this could be the start of a new big threat against them, since Annabeth was very important in their strategies and battles.
Definitely the Giants. They have a history of causing trouble for the demigods. Annabeth's disappearance could be part of their plan to weaken the demigod forces. They might think that without Annabeth, the demigods would be less organized in their battles.
In this fanfiction, it might be that Percy goes missing because he is on a self - imposed mission. He could be trying to protect his friends from a danger he discovers, and he doesn't want to involve them, so he disappears without a trace. He might be in a far - off land, facing challenges all alone.
Well, one possibility is that they'll have to deal with adjusting to living together. Maybe there will be arguments over little things like who does the dishes or how to decorate their place. Another could be that they discover new things about each other that they didn't know before, which deepens their relationship.
The plot could involve Percy arriving early to camp and immediately noticing that something is wrong because Annabeth is missing. As he searches for her, he stumbles upon evidence that she has been in a battle with a powerful new enemy. When he finally finds her lifeless body, it shatters him. He then decides to train harder than ever before to avenge her death. He forms a small group of trusted friends from the camp and they set out to track down the enemy responsible, all while Percy is struggling with his grief and guilt for not being there to protect her.
Percy might be a source of comfort for Annabeth. School can be a tough place sometimes, with all the academic pressure and social drama. Percy is someone she can always rely on, and without him, she feels less secure. Maybe he has a unique way of making her laugh or feel better when she's down, and she misses that a great deal.
There could be all sorts of fanfictions. Maybe it's a story of her frantic search or her dealing with her emotions during his absence.
Well, it could lead to a lot of drama. Maybe they get caught by their friends who are shocked at first but then gradually accept it. Their relationship might become more public and they have to deal with the reactions of others.
One possibility is that it could lead to some awkwardness between them for a while. They might avoid each other's eyes and have stilted conversations until they finally talk about what happened and clear the air.
There could be a school janitor who is actually an old demigod who retired from the hero business. He could give Percy some valuable advice based on his own experiences. And of course, Percy will surely meet Annabeth's classmates, some of whom could be friendly and some not so much, depending on how they react to the new boy in school.
A new character introduced in the fanfiction might be the main suspect. This character could have some ulterior motives that are not clear at first. They might seem friendly but are actually behind Percy's disappearance. For example, they could be working for an evil organization that wants to use Percy for some dark purpose. So they plan his disappearance carefully to achieve their goals.