They could go on a journey to find a lost demigod. Percy uses his navigation skills in water and Piper uses her ability to communicate with people to track down the missing person. Along the way, they face various challenges and form a deeper friendship.
One idea could be that they go on a quest together and discover a new type of magic that only they can control because of their unique powers. This new magic could be the key to saving their world from a great threat.
A great plot could be that Percy and Piper are the only ones who can see a strange new threat that is slowly creeping into their world. They initially don't believe each other but then realize that they are right. So they start investigating together. Along the way, they learn more about their shared heritage that has been hidden from them, and this new knowledge helps them face the threat.
One idea could be that Percy and Piper have to team up for a special quest that involves both water and charm powers. Maybe they need to retrieve a magical artifact that is hidden in a place where water and wind elements combine, like a floating island in the middle of a tempestuous sea. They would have to use Percy's ability to control water and Piper's power to charm the creatures guarding the island.
One interesting plot could be Piper using her charm - speak to help Percy in a really tough battle against some new and powerful monsters. Maybe she has to charm - speak a whole army of them to make them fight each other instead of Percy.
One idea could be a quest where Percy, Jason and Piper have to team up to find a lost artifact that has the power to control the elements. They face various mythical creatures along the way and have to use their combined powers to overcome obstacles.
One common plot idea is a quest that both Piper and Percy go on together. Maybe they have to retrieve a powerful artifact that has been lost and is threatening to disrupt the balance in the world of the series. Another idea could be a love triangle situation where Piper, Percy, and another character are involved in a complex web of emotions.
Another plot could be that Amphitrite tasks Percy with protecting a special part of the ocean that is home to some endangered sea creatures. Percy then has to use his powers and wits to fend off various threats, like illegal fishermen or sea - polluting entities. This would show a more professional relationship between them.
A more character - driven idea could be that Percy Piper is feeling down and he finds comfort in baking lemon - flavored treats. Through this process, he learns more about himself and his relationships with others.
One possible plot idea could be that Piper struggles with her feelings. She might try to deny them at first because Percy is in a relationship or because of loyalty to her friends. Another idea is that there could be some sort of magical influence that makes her think Percy is hot, and she has to figure out how to break free from it while also dealing with her new - found attraction.
Another plot could be set in an alternate universe where Piper and Jason have different roles. For example, Piper could be a powerful sorceress and Jason a knight, and they have to team up to defeat an evil dragon that is terrorizing the kingdom. This would allow for a fresh take on their characters and relationship.