A great plot would be Percy being ostracized by some of the other demigods because of his primordial heritage. He would have to struggle not only with external threats but also with finding his place among his peers. This could lead to him forming new alliances with unlikely characters and ultimately proving his worth.
Another plot could involve Percy's relationship with the primordial beings. Maybe he has to seek advice from them to save the world from a greater threat. He could learn about ancient secrets and prophecies that only the primordials know, which would add an interesting twist to the story.
One idea could be Percy discovering new powers related to his primordial heritage. For example, he might be able to control some ancient elements that no one has seen before. Another plot could involve Percy having to face off against other demigods who are jealous of his unique parentage. They might try to undermine him or even start a feud.
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 exploring the backstory of the eldest primordial in more detail. Maybe how it came to be and its relationship with the other primordial beings. Another could be Percy having to team up with the eldest primordial to face a new, greater threat that neither could handle alone. For example, a powerful ancient evil that is awakening.
One plot idea could be Percy having to face the Primordial of Destruction in a direct confrontation. Maybe the Primordial is awakened by an ancient evil spell and starts causing chaos in the demigod world. Percy has to gather allies from different camps and figure out a way to stop it before it destroys everything. Another idea is that the Primordial somehow infects Percy with a part of its destructive power, and he has to struggle not to let it take over him while also trying to find a cure. And a third plot could involve the Primordial creating illusions of destruction all over the world, and Percy has to travel through these illusions to find the real source and defeat it.
One idea could be Percy having to time - travel back to the very beginning when the primordials first emerged. He could accidentally disrupt the balance of power among them, leading to all kinds of chaos in the present. For example, if he stops a primordial from creating a certain element that is crucial for the modern world's magic. Another plot could involve Percy teaming up with a primordial of time to fix time - loops that are causing disasters in different eras. Maybe he has to enter the time - realm of the primordial, which is a completely different and dangerous place filled with temporal anomalies.
One idea could be that the Primordial King emerges from a long slumber and Percy has to figure out a way to either ally with him or stop him from wreaking havoc on the demigod world. Maybe the Primordial King has some ancient knowledge that could help Percy in his fight against the Titans or other enemies, but at a great cost.
You could also write a fanfiction where the primordial gods start to reawaken in the modern world. Percy has to figure out how to deal with them while also protecting his friends and Camp Half - Blood. The primordial gods' powers and motives are very different from the Olympian gods he is used to, so it would be a huge challenge for him. He might have to learn new magic or tap into his own latent abilities that he didn't even know he had.
One idea could be Percy having to hide his true parentage from the other demigods at Camp Half - Blood. He's constantly worried about being shunned or attacked if they find out. Another could be Percy using some of Medusa's powers in a unique way, like being able to turn enemies to stone briefly but with some unforeseen consequences. Maybe he has to go on a quest to find a way to control these new powers.
One idea could be Percy having to navigate the Underworld politics due to his relation to Hades. He might face challenges from other underworld creatures who don't trust him. Another could be Percy using his newfound powers as the son of Hades to fight against an ancient evil that threatens both the mortal and underworld realms. Or perhaps he has to deal with the stigma of being Hades' son in the demigod community and prove himself worthy.
A possible plot for a Percy Son of Pluto fanfiction could be that Percy is haunted by strange visions that seem to be related to Pluto's past. He decides to embark on a journey to find out the truth behind these visions. As he delves deeper into the mystery, he finds out that there is a forgotten prophecy about him that could change the balance between the gods and the demigods. He then has to gather allies, both from the surface world and the Underworld, to face whatever is coming.