One idea could be Percy having to face a new and powerful enemy that is secretly being controlled by an ancient god who has a grudge against the Olympians. He'd have to use his unique powers and the help of his friends in unexpected ways.
A plot could revolve around Percy being falsely accused of stealing a powerful artifact from the Hunters. He then has to prove his innocence while also protecting the Hunters from those who really stole the artifact and are now using it to target the Hunters. He teams up with some of the more skeptical Hunters who initially thought he was guilty, and together they uncover the real culprits.
A great plot idea is Percy being chosen by Artemis to be a special liaison between the Hunt and Camp Half - Blood. This would create a lot of interesting interactions as he tries to balance the needs and cultures of both groups. He could also face challenges from within both camps as some might not trust his new role. Also, a plot where Percy discovers a lost relic that is important to the Hunt and has to go on a quest with the Hunters to restore it could be really engaging.
A cool plot could be Percy getting lost in a different dimension that is related to the Greek mythology but has been forgotten. In this dimension, he has to find his way back while also helping the lost souls there. He could learn new powers and make new allies that would be useful when he returns to his own world.
One idea could be Percy having to protect Artemis from a new and powerful monster that has a special ability to track her. Another could be Percy and Artemis teaming up to retrieve a lost magical artifact that is important for the balance of the world. Maybe Percy has to enter Artemis's domain in the forest and face challenges there while protecting her.
One idea could be exploring a new prophecy that involves Percy and his friends having to team up with some of the lesser - known gods. This would introduce new characters and powers into the story.
One idea could be Percy having to face a new set of challenges in the world of the gods as a consort. Maybe he has to deal with the politics among the deities, like trying to broker peace between feuding gods. Another could be Percy using his new position to help demigods in a more powerful way, for example, getting better training facilities for them.
Perhaps Percy is chosen by the gods to be a mediator in a great council. He has to deal with the bickering and power - plays of the gods. He uses his experiences with both the mortal and divine worlds to bring some order. Along the way, he uncovers secrets about the gods' past that could change his relationship with them forever.
A possible plot could be that Percy is summoned to Japan by the Shinto gods to help stop an ancient evil that is threatening both the Greek and Shinto worlds. This evil could be a powerful demon that has the ability to disrupt the balance between the two pantheons.
One idea could be Percy having to team up with an Egyptian demigod to stop a common enemy that is causing chaos in both Greek and Egyptian mythological realms. Maybe a powerful monster that is using the powers of both mythologies to grow stronger.
Perhaps Percy is framed for a crime against the gods and has to fight them while also trying to prove his innocence. He could travel to different realms in the process, seeking evidence and allies. Along the way, he might learn some dark secrets about the gods that could change his perspective on them forever.