One idea could be Percy having to face a new type of monster that is a combination of all the previous ones he's fought. This super - monster could be created by an ancient and powerful enemy who wants to truly become the king of monsters. Another plot could involve Percy having to enter the realm of the dead to find a weapon that can defeat the king of monsters. Maybe he has to make a deal with Hades to get this weapon.
Perhaps a plot where the she - monsters have a special power that only Percy can unlock or control. For instance, they hold the key to a powerful weapon, and Percy has to earn their trust to access it. This would involve Percy going through a series of trials set by the she - monsters.
One idea could be Percy having to face a new type of monster that is a hybrid of multiple known ones. This would test his skills in new ways.
One idea could be exploring the origin of the Primordial of Monsters in more depth. Maybe there's a hidden prophecy related to it that Percy and his friends have to uncover. Another could be a story where the Primordial of Monsters forms an unexpected alliance with Percy against an even greater threat.
One idea could be Percy discovers a hidden underwater kingdom where he is hailed as a long - lost king. He has to deal with the politics and power struggles there while also protecting his friends from the new threats this kingdom brings.
A cool plot could involve Percy as a super soldier being sent on a secret mission by the gods. This mission might be related to a new threat that combines the mortal world's technology and the mythological realm. He would have to use his super soldier skills, like enhanced strength and combat training, along with his demigod powers to succeed.
One unique plot idea could be that Percy discovers he has a special connection to the dragons because of his demigod powers. Maybe he can communicate with them on a deeper level than others. Another idea is that the dragons are the key to solving a new threat to the demigod world, like a powerful ancient evil that only the combined strength of the demigods and the dragons can defeat.
One idea could be Percy having to protect his harem from a new and powerful monster threat that specifically targets them because of their connection to him. Another could be Percy exploring his relationships within the harem while also dealing with his identity as the son of Poseidon in a world full of monsters.
One idea could be that the Phoenix King has a connection to an ancient prophecy in the Percy Jackson world. Maybe he is the key to saving Camp Half - Blood from a new, powerful enemy. For example, his powers of rebirth could be used to revive a fallen god who is crucial in the fight against the enemy.
One plot idea could be Percy having to deal with political intrigue in Atlantis. Since he's the king, different factions might try to influence him or even overthrow him. Another could be Percy using his powers to explore uncharted areas of Atlantis and discover ancient secrets. Maybe he finds a powerful artifact that has the potential to either save or destroy Atlantis.
Well, perhaps in the fanfiction, the Hellhound King has a connection to an ancient prophecy. Percy is tasked with either killing it or taming it to fulfill the prophecy. As Percy delves deeper, he realizes that the Hellhound King has a complex personality and motives that are not just about destruction. It could also be that the Hellhound King has a hidden lair full of secrets and treasures that Percy needs to explore for some reason, like to find a weapon to defeat a new threat.