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.
Well, a cool plot for 'Percy Jackson and Medusa fanfiction' might be that Medusa is not truly evil, but was misjudged. Percy gets to know her side of the story, like how she was turned into a monster against her will. Then he has to help her fight against the real villain who wants to use her power for bad. Also, perhaps Percy accidentally steps into a magical trap set by Medusa, and to get out, he has to complete a series of challenges she sets for him, all while learning more about her past.
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 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 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.
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 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 plot idea could be Percy having to team up with Hera to rescue a group of demigods who have been captured by an ancient monster. Hera, despite her usual aloofness, realizes she needs Percy's unique skills.
One idea could be that the gods have lost their powers due to some ancient curse, and it's up to Percy and his friends to find a way to restore them before the apocalypse fully hits. Maybe they have to journey to the very origin of the gods' power.
Well, a cool plot might be that there's a merman who has a special power to control water currents, and Percy needs his help to find a lost magical artifact deep in the ocean. In return, Percy has to help the merman get accepted by the surface - dwelling gods. Or perhaps a merman is wrongly accused of stealing Poseidon's trident and Percy has to prove his innocence.
One idea could be Percy having to face a new threat underwater that only Poseidon can help him understand. Maybe a lost civilization that Poseidon once had a connection with is now causing trouble. Another could be Percy going on a quest to find a powerful artifact that was once Poseidon's but has been stolen, and Poseidon gives him special powers or guidance along the way.