A plot could involve Percy and his friends getting separated during a vapor travel accident. Each of them has to find their own way back while dealing with various challenges specific to the places they end up in. For example, Annabeth might find herself in a library - like realm where knowledge is a weapon and she has to outwit a knowledge - stealing creature. Percy could end up in a water - based vapor world where the laws of physics are different and he has to adapt quickly to survive.
Perhaps there's a prophecy related to vapor travel. Percy has to embark on a journey through different vapor - filled regions to find an ancient artifact that can save the world from a new threat. Along the way, he meets old enemies who have also discovered the power of vapor travel and are using it for their own evil purposes.
One interesting plot idea could be Percy traveling back in time to the era of the Greek gods' creation. He could witness the birth of the first gods and goddesses and perhaps even play a role in some of the early events. Another idea is him going back to the time of a great hero like Hercules and joining forces with him to complete a quest that was left unfinished. Or he could travel to the future to see what has become of the demigod world and then come back to the present to try and prevent a bad future from happening.
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 interesting plot idea could be Percy traveling back in time to the era when the gods first came to power. He could witness the creation of the Olympian council and get involved in the early power struggles among the gods. Another idea is that a god sends Percy on a time - travel mission to fix a mistake in the past that is now threatening the present world of demigods. For example, if a powerful magical artifact was lost in the past and its absence is causing chaos in the present.
A group of rogue demigods who have been using vapor travel to steal from other realms. They could be both enemies and potential allies depending on the situation. They might have unique powers related to vapor manipulation that Percy and his friends have never seen before.
One idea could be a crossover where Harry and Percy meet during a magical event that blurs the boundaries between their worlds. Maybe a powerful dark force is causing chaos in both the wizarding world and the demigod world, and they have to team up to stop it. They could share their unique magical abilities and learn from each other's fighting styles.
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.