One idea could be Percy having to face a series of challenges set by Nemesis that test his sense of fairness. For example, he might be put in situations where he has to choose between helping his friends or following the rules of a very strict game. Another could be Nemesis creating doppelgangers of Percy's friends to make him question who he can trust.
A plot idea could involve Nemesis getting angry because Percy has been getting too much good luck lately. So, she decides to turn his life upside down. She might start by making his powers go haywire in the middle of a battle with a new, powerful monster. This would force Percy to rely more on his wits and the help of some unlikely allies. Maybe he has to team up with some minor gods who are also on Nemesis' bad side.
One idea could be Percy having to face a series of challenges set by Nemesis that test his sense of fairness. For example, he might be put in situations where he has to choose between helping a friend who has made a mistake or following the strict rules. Another could be Nemesis creating an illusion of a perfect world for Percy, but he has to figure out that it's all a ruse and break free.
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.
A great plot could involve Percy being sent on a quest by a primordial. Since primordials are so powerful and ancient, the quest would likely be extremely dangerous and full of unknown challenges. For example, he might have to retrieve an ancient artifact that was lost in the time before the gods. This could lead to Percy facing not only physical challenges but also moral dilemmas as he deals with the primordial's motives.
A plot could involve Hera taking Percy on a journey through the past to show him the origins of the gods' power and how it relates to the current problems in the demigod world. This would give Percy a new perspective and knowledge that he can use to save the day. Or perhaps Percy accidentally stumbles upon a secret of Hera's that she's been hiding for centuries, and she has to enlist his help to keep it safe, all while other gods are trying to uncover it too.
One idea could be Percy gets cursed by Circe to forget his friends and mission, and he has to struggle to regain his memories while under Circe's influence. Another could be Circe teams up with Percy to fight a common enemy that threatens both the mortal and magical worlds. Or perhaps Percy has to rescue his friends from Circe's island after they are turned into animals, but he discovers that Circe has a more complex backstory and motives than he thought.
A great plot could involve Percy going on a quest not for the typical reasons like saving the world or a camp, but to find a way to restore the memories of a friend who has lost them due to a powerful spell. Along the way, he discovers secrets about his own past that were hidden from him. He might also encounter new demigods with unique powers who either help or hinder his journey.
One idea could be that Padme somehow gets transported to the Percy Jackson universe. She could use her diplomatic skills to help resolve a conflict between the demigods and a new, powerful enemy. Maybe she teams up with Percy and they combine their unique abilities to defeat the foe.
One idea could be Percy getting a magical haircut that gives him new powers. Maybe the hair is cut by a special sword and as each strand is cut, a new ability like controlling water in a more unique way or being able to communicate with sea creatures better is bestowed upon him.
Another plot idea could be that Percy discovers a hidden underwater civilization that is not related to Poseidon. This civilization has its own set of rules and powers. Percy has to navigate through their complex society and politics while also trying to convince them to help the demigods in a coming war against an ancient evil that is rising again. The underwater civilization could have unique magic and technology that the demigods have never seen before.
One idea could be Percy and his friends discover a hidden underwater section in the aquarium that is a portal to a lost underwater civilization. Another could be that the aquarium animals start to communicate with Percy due to his demigod powers and ask for his help to solve a mystery within the aquarium. Maybe there's a cursed object in the aquarium that is causing strange phenomena and Percy has to find and remove it.