Well, perhaps in the fanfiction, Annabeth betrays Percy's trust in a relationship way. She could start seeing someone else and use Percy's secrets against him to make herself look better in front of the new person. Or she might side with a group that Percy is against, believing their lies and false promises over Percy's warnings. This could lead to Percy feeling completely alone and having to find new allies.
In some fanfictions, it could be that Annabeth is influenced by a powerful and evil force. This force convinces her that Percy is a threat to their world. So, she decides to betray him by stealing his powerful weapon, like Riptide. Without his weapon, Percy is at a great disadvantage. He then has to embark on a journey not only to get his weapon back but also to prove his innocence and that he is not the threat Annabeth believes him to be. Along the way, he might meet new friends who help him face this difficult situation.
One possible plotline could be that Percy and Nico are betrayed by a close friend within the demigod community. Maybe they are framed for a crime they didn't commit, like stealing a powerful artifact from the gods. This could lead to them being hunted by other demigods while they try to clear their names.
Another plotline could be that they are babysitting a group of demigod toddlers at Camp Half - Blood. The toddlers keep getting into mischief, like stealing nectar and ambrosia. Percy and Annabeth have to keep them in line, teach them about the rules, and also make sure they don't accidentally harm themselves with their budding powers. It could be a really cute and chaotic story.
Well, it could start with Percy being framed for a crime he didn't commit by the gods. This false accusation makes him feel extremely betrayed. He then has to go on the run while trying to prove his innocence. Along the way, he meets other outcasts and together they form a resistance against the gods. They might try to find a way to hold the gods accountable for their actions and create a new order where demigods are treated more fairly.
One possible plotline could be that Percy discovers his so - called friends have been secretly working against him all along. Maybe they spread false rumors about him, which makes everyone in his world turn against him. He then has to go on a journey alone to clear his name.
Perhaps in the fanfiction, Percy is influenced by some dark magic or a curse that makes him betray the son of chaos. The son of chaos then has to decide whether to forgive Percy or seek revenge. The story could also explore how their friends and allies are affected by this betrayal and whether they side with Percy or the son of chaos.
Perhaps Annabeth is wrongly accused of a crime and is made a slave as punishment. Her friends, including Percy, try to prove her innocence while also dealing with the internal politics of the demigod society that allows such a cruel situation to exist. This could involve a lot of character development for all involved as they question the fairness of their world's laws and norms.
In this fanfiction, a plotline could involve Percy having to introduce Annabeth to his side of the family. This would be really interesting as it would show a whole new aspect of Percy's world to Annabeth. Another plotline could be Annabeth getting into trouble at school or with other demigods, and Percy having to step in as her father to resolve the situation. This would show how he uses his influence and authority in this new role. There could also be a plotline where Percy and Annabeth find out that there's a prophecy related to their new relationship. They would then have to figure out what it means and how to fulfill or avoid it, which would add a lot of tension and mystery to the story.
A possible plotline could be a mortal who becomes friends with Percy and Annabeth without knowing their true nature at first. As the story progresses, they start to notice strange things about their friends. Another plotline could involve a mortal who is caught in the middle of a demigod battle. They have to rely on Percy and Annabeth to keep them safe while also trying to understand what's going on.
In some fanfictions, this scenario could be used as a turning point in their relationship. Percy seeing Annabeth in such a vulnerable state could make him realize his deeper feelings for her. He starts to see her not just as a friend and teammate, but as someone he has strong romantic feelings for. This could lead to a series of events where he tries to court her in his own clumsy way.
One possible plotline could be that Percy is under some sort of mind control. Maybe a new villain has the power to manipulate his thoughts and actions, making him turn against Annabeth. For example, the villain could use Percy's insecurities about his past mistakes to drive a wedge between them. He might start by leaking some of Annabeth's secret plans to the enemy, causing her to be in danger without realizing Percy is the source.