One idea could be that Percy is kidnapped by a group of rogue demigods who want to use his powers for their own gain. They might try to force him to lead them to hidden treasures or secrets in the demigod world. Another plot could involve Percy being kidnapped by a new, unknown enemy from an ancient myth that has been awakened and sees Percy as a key to its revival. Or perhaps he's kidnapped by a powerful sorcerer who wants to steal his connection to Poseidon for dark magic purposes.
In a fanfiction where Percy Jackson is kidnapped, it could start with Percy being lured into a trap by a seemingly friendly figure. This figure could be an agent of an old enemy, like Kronos, who has some remnant power or followers still. Once kidnapped, Percy finds himself in a strange place, cut off from his friends and allies. His kidnappers might be trying to break his spirit so that they can turn him against his friends. Percy would then have to rely on his wits and the small amount of power he can muster in captivity to find a way to escape and send a signal to his friends for help.
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.
One idea could be that Percy discovers a hidden realm within the world of the Wanderer. This realm has its own set of rules and challenges that he has to navigate. Maybe he meets new demigods there who have been lost for centuries.
One idea could be Percy having to make a deal with a minor underworld god to save Annabeth from a new and strange curse that's slowly draining her life force. He has to complete a series of tasks in the underworld that test his loyalty and courage more than ever before.
Well, perhaps the shield was once owned by a powerful demigod from the past. In the fanfiction, Percy starts having strange dreams about this demigod and the shield's true origin. As he investigates, he realizes that the shield holds the key to defeating a new, emerging evil that threatens not just Camp Half - Blood, but the entire world of gods and mortals. So, he has to figure out how to unlock the shield's full potential.
Here's an idea. Emma could be a mortal who accidentally stumbles upon the world of demigods. Percy is tasked with protecting her while also trying to keep the secret of the demigod world from getting out. As time passes, Emma shows an uncanny ability to understand the demigod world, almost as if she has some latent demigod blood in her. They develop a bond as they face various monsters and challenges sent by the gods who are not happy with a mortal being so involved. Their relationship could be a mix of friendship and something more as they go through these experiences together.
One interesting plot idea could be a crossover where Percy Jackson's demigod powers interact with the magic in The Originals. For example, his ability to control water could be used in a battle against the vampires in New Orleans. Maybe he helps the Mikaelson family during a flood caused by a powerful witch, using his water - based powers to save them and in turn they help him with some demigod - related problem he has, like finding a lost artifact.
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.
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.