A cool plot could be that the ancient prisoner is part of an ancient pact that the gods made and forgot about. Percy stumbles upon the mystery of the prisoner and realizes that if he doesn't free the prisoner in time, all of the demigods will be in danger. As he tries to free the prisoner, he has to face various challenges set by the gods who don't want the pact to be broken, and also deal with the fact that the prisoner may not be as trustworthy as he seems at first.
Perhaps the ancient prisoner is actually a wronged hero from the past. Percy discovers this and has to fight against the forces that want to keep the prisoner locked up, all while dealing with the internal politics of the demigod world. The prisoner could hold the key to a new kind of magic or weapon that the demigods need to defeat an upcoming evil. It could also be that the prisoner has a connection to Percy's family history, and by freeing him, Percy learns more about his own heritage and powers.
It could be an old Titan who was locked away by the Olympians. Maybe one who was not as evil - minded as the others and was wrongly imprisoned.
One idea could be Percy having to face a new and powerful sea monster that is awakened by an ancient curse. He'd have to use his powers in unique ways, maybe even discovering new abilities as the son of Poseidon. Another could be Percy traveling to a long - lost underwater civilization that has a connection to his heritage as an ancient son of Poseidon.
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 plot could be that Sally Jackson plants a lemon tree in her garden that turns out to be a gift from a minor goddess. The lemons from this tree have the power to give those who eat them the ability to see through illusions for a short time. Percy uses these lemons to his advantage when he is on a mission to retrieve a powerful artifact that is guarded by a monster that uses illusions to protect it.
Perhaps a fanfiction where Percy and Thalia are transported to an alternate dimension. In this new world, they have to figure out how to get back while also dealing with different versions of their friends and enemies. Thalia might have to face an alternate version of herself who has turned evil, and Percy could be the one to help her see the light again. It would be full of action, drama, and self - discovery.
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.
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 that Percy has to face a new threat from Medusa's long - lost sister who has even more powerful petrifying abilities. He has to use his skills and some newfound knowledge about the Gorgon lineage to defeat her.
Perhaps there's a situation where a powerful deity wants to pit Percy and Clarisse against each other in a tournament. But instead of fighting, they decide to work together to outwit the deity. They train in secret, combining their different fighting styles, and in the process, they form an unexpected bond.
One plot idea could be Hera going on a quest with Percy. Since she's a goddess and Percy is a demigod, their dynamic would be really interesting. They could face challenges that test both Hera's divine powers and Percy's wits as a hero. Another idea is a story where Hera loses her memory and Percy has to help her remember who she is while protecting her from other threats in the demigod world. Maybe some monsters would see her in a weakened state as an easy target.