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.
A plot idea might involve Percy meeting other children of monsters. They form an unlikely alliance to fight against a common enemy that threatens both the demigods and the monsters. Since he is the son of Medusa, Percy could have a special connection to the world of monsters that other demigods don't. He could use this to broker peace between the two sides in the end.
Perhaps Percy discovers that there is a prophecy about him being the son of Medusa. The prophecy says that he will either bring great destruction or be the key to saving both the mortal and the mythological worlds. As he tries to figure out his role, he faces many challenges and has to make difficult choices along the way.
One idea could be a time - travel plot where Percy goes back in time and meets a younger, different Medusa before she was turned into a monster. Another could be a story where Medusa somehow gets a chance at redemption and Percy has to decide whether to trust her or not. Maybe a plot where they have to team up to defeat a greater evil that threatens both of them.
One idea could be Percy has to face Medusa again, but this time she has an army of stone minions. Another could be that Medusa was actually cursed by a more powerful god and Percy has to find a way to break it. Maybe Percy discovers Medusa has a long - lost relative who is being held hostage and he has to make a deal with her.
One idea could be that Percy has to face a new threat from Medusa's long - lost sister who has even more powerful petrifying abilities. He has to use his skills and some newfound knowledge about the Gorgon lineage to defeat her.
In a fanfic, perhaps Percy and his friends accidentally stumble upon Medusa's lair while on a quest. But instead of immediately attacking, Medusa offers them a deal. She says she will give them a valuable clue to complete their quest if they help her get revenge on one of the minor gods who wronged her in the past. Percy and his friends then have to decide whether to trust Medusa or not.
One interesting plot could be Percy having to face off against Medusa's other children in a battle of wits and strength. Maybe they are out to prove themselves as worthy as Percy, who has the fame of being Poseidon's son. Another could be Percy having to deal with the stigma of being 'Son of Medusa' in a world that fears her so much. He might have to constantly prove he's not a monster like his 'mother' is perceived.
One idea could be Percy going on a quest that was pre - ordained by the Fates. He discovers that his every move is part of a larger plan they have for the demigod world. For example, he might be sent to retrieve an ancient artifact that holds the key to maintaining the balance between the gods and the Titans, and along the way, he has to face challenges that test his loyalty to the Fates' plan.
Well, a great plot for Percy son of Neptune fanfiction could involve Percy getting lost in a time - warp within the ocean. He could end up in an era where Neptune was just starting to gain his powers. This would force Percy to interact with a younger version of his father and also deal with the different rules and threats that existed back then. Additionally, Percy could find a way to bring some knowledge from that time to his present to help in a current conflict. Also, Percy might have to team up with some primitive sea creatures who have their own unique abilities and motives.
One idea could be Percy having to prove himself to Zeus. Since he's his son in this fanfiction, Zeus might be overly critical. Percy could go on a quest to retrieve a powerful artifact that was once Zeus's but was lost. Another plot could involve Percy's relationships with other demigods changing. They might be jealous or in awe of his new status as Zeus's son.
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.