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.
Another plot could be a time - travel story. Piper accidentally sends Percy back in time to a period in Greek mythology. Percy has to figure out how to get back to his own time while Piper, in the present, is trying to find a way to bring him back. Along the way, Percy discovers secrets about his own powers and Piper learns more about the roots of her charm - speaking abilities.
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 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.
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 interesting Percy Jackson Piper fanfiction could be 'Piper's New Adventure'. In it, Piper discovers a hidden power within herself that she has to learn to control while Percy is off on a quest. It's full of action and character development.
One plot idea could be Percy having to team up with Hera to rescue a group of demigods who have been captured by an ancient monster. Hera, despite her usual aloofness, realizes she needs Percy's unique skills.
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.
One idea could be a fanfic where Percy and Piper team up to rescue a lost demigod. They could use their combined powers, Percy's water - based powers and Piper's charm - speak, to overcome obstacles in a dangerous forest. Another idea is that they are on a quest to find an ancient artifact, and they have to solve a series of puzzles together. Maybe they are in a temple filled with traps, and they rely on each other's skills to survive.
One idea could be Percy and his friends getting trapped in a virtual game world that's a twisted version of their own. They have to use their demigod skills to defeat the digital monsters and find a way out.