One possible storyline could be Percy having to face Medusa in a new way that doesn't involve just slaying her. Maybe he discovers that Medusa has a side to her that was misunderstood all along. For example, she was cursed not out of pure evil but due to the jealousy of the gods. Percy could then try to find a way to break her curse, leading to an unlikely alliance between them.
Another storyline could be a time - travel fanfiction. Percy somehow gets sent back in time and meets Medusa before she was fully turned into the monster she is known as. He tries to prevent the events that led to her transformation. This could involve him interacting with a younger, more innocent Medusa and trying to change the course of her fate, while also dealing with the consequences of meddling with the past in the world of Percy Jackson.
One storyline could be that Percy, while trying to defeat the Nemean Lion, discovers that it has a more sentient side. The lion, instead of attacking him immediately, seems intrigued by Percy. As they interact more, a bond forms that slowly turns into a romantic connection. Percy realizes that the lion is not just a mindless beast but has been misunderstood all this time.
One storyline could be their child discovers a new, hidden power within themselves. Maybe they can communicate with a long - forgotten ancient Greek deity that has been dormant for centuries. Another could be the child getting lost in the Labyrinth while trying to prove themselves worthy to their parents. They meet all sorts of strange creatures and have to use their wits, just like Percy and Annabeth did.
One idea could be Percy having to face Medusa in a new way that challenges his usual methods. Maybe he gets trapped in a dream where Medusa has the upper hand and he has to use his wits more than his sword. Another could be a scenario where Medusa is somehow redeemed and teams up with Percy against a greater evil. For example, an ancient monster that threatens both the mortal and mythological worlds.
One possible storyline could be a crossover where Naruto's world collides with that of Percy Jackson and Hera somehow gets involved. Maybe Naruto is transported to the Percy Jackson universe and has to deal with Hera's schemes while also trying to find a way back home. Another could be that Percy and Naruto team up to face a threat that Hera has foreseen, and they must learn to combine their powers and skills. For example, Naruto's ninjutsu and Percy's demigod abilities working together to defeat a powerful monster.
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 popular storyline could be Percy and Annabeth going on new adventures together, exploring different parts of the mythological world that weren't covered in the books. For example, they could travel to a long - forgotten island full of ancient and powerful magic.
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 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.
One idea could be Percy using his water powers in a unique way against Medusa. For example, he could create a water shield that reflects Medusa's petrifying gaze back at her in a fanfiction. Another could be a scenario where Medusa, instead of being an enemy, teams up with Percy to face a greater threat.