Here's another idea. The Shinto gods decide to test Percy to see if the Greek demigods are worthy of an alliance. They send him on a series of trials that are designed around Shinto beliefs and values. Percy has to use his wits and powers to pass these trials while also dealing with the politics and rivalries among the Shinto gods.
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.
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 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 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.
A possible plot for this fanfiction is that the curse makes Percy lose his memory bit by bit. He forgets his friends, his past adventures, and even his own identity. As he struggles to figure out what's wrong with him, he starts having strange visions that are both a clue to the curse and a danger in themselves. Maybe the gods who cursed him are trying to erase him from existence, and he has to rely on his instincts and the few remaining memories to fight back. He might meet new characters who are either trying to help him or are part of the gods' plan to keep him cursed.
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.
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.