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.
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.
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.
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 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.
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.
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.
One idea could be a story where Percy has to team up with a minor Greek god, like Hypnos, to defeat a new threat that's causing nightmares across the demigod world. Another could be a fanfic exploring what would happen if Percy was sent back in time to the age of the original Greek heroes and had to prove himself among them.
Well, a great plot could involve Percy being transported to the Norse realm and having to prove himself to Odin. He might have to participate in a series of trials that are designed for Norse heroes. Or perhaps he gets caught in a feud between different Norse factions and has to use his wits and powers to bring peace. He could also discover a connection between the Greek and Norse pantheons that no one knew about before.
One idea could be Percy having to face a new, powerful enemy that is a threat to his newfound position as king of the gods. This enemy could be a primordial being that has been awakened. Another plot could involve Percy trying to reform the rules among the gods, which causes a lot of internal conflict as the other gods resist change. And perhaps Percy discovers a long - lost prophecy related to his kingship that changes everything he thought he knew.
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.