A plot could involve Percy being tricked into using the power of darkness to save his friends, but then he has to face the consequences as it starts to corrupt him. He then has to find a way to purify himself and break free from the dark influence.
Perhaps in the fanfiction, Percy is transported to a realm ruled by the Prince of Darkness. He has to navigate this new and dangerous world, make allies among the dark creatures, and find a way to either defeat the Prince of Darkness or change the nature of this dark realm for the better.
One idea could be Percy discovers he has a hidden connection to the powers of darkness that dates back to an ancient prophecy. Maybe he has to learn to control this new aspect of his powers while still being true to his friends at Camp Half - Blood.
One idea could be Percy exploring his new powers as the 'Son of Darkness'. He might discover abilities that are the opposite of his normal powers, like creating shadows instead of controlling water. Another could be his struggle to fit in with his friends and the demigod community when they view him as different now. He could also face new enemies who are attracted to his new dark nature.
One idea could be Percy having to navigate the complex politics of Hell while trying to maintain his own moral code. Maybe he has to make alliances with various demons to stop a greater evil from escaping Hell and wreaking havoc on the mortal world.
One idea could be Percy facing a new type of monster that is immune to water. This would really challenge his typical combat style. Another could be Percy having to team up with an old enemy to defeat a greater threat that endangers both the mortal and demigod worlds. Maybe he has to journey to a completely unknown realm within the Chaos to retrieve a powerful artifact that is the key to saving his friends.
Well, you could have a plot where Percy is sent on a quest to find a powerful artifact that has the ability to control all sea currents. Along the way, he meets a group of pirates who are also after the artifact for their own greedy reasons. He has to outwit them and deal with the challenges of the deep sea, like deadly whirlpools and giant sea serpents, all while trying to keep the artifact out of the wrong hands.
A plot could involve the Prince of Darkness in a battle against other supernatural forces. He has to team up with unexpected allies, like a group of young witches who are also trying to protect their world from a greater evil. This would lead to a lot of character development as they learn to work together.
One plot idea could be that Percy Jackson, while on a quest, accidentally stumbles into Diana Prince's world. They team up to fight a common enemy that threatens both the demigod and the Amazonian worlds. Maybe a powerful ancient evil that has the ability to disrupt the balance between the mortal and the divine in both their realms.
One idea could be Percy having to face a new underwater threat that is specifically targeting Atlantis. Maybe a powerful sea monster that was once imprisoned by the Atlanteans has been released and Percy has to figure out a way to stop it without causing too much destruction to the underwater city.
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 Percy and his friends getting trapped in a virtual game world that's a twisted version of their own. They have to use their demigod skills to defeat the digital monsters and find a way out.
Maybe Percy is sent on a quest to Atlantis because there are rumors of a traitor among the Atlanteans who plans to side with the Titans. Percy has to navigate the complex political situation in Atlantis and figure out who the traitor is before it's too late. He also has to deal with the Atlantean magic which is different from what he's used to.