Perhaps Percy is held captive by a primordial force that has been awakened from a long slumber. This force is angry at the gods and sees Percy as a pawn. The place where he's held captive is a dimension that's constantly shifting and changing, making it hard for him to find a way out. He has to communicate with the spirits of the place to learn how to control the shifting and find the weak spot in the prison. Meanwhile, his friends back at Camp Half - Blood are trying to figure out how to reach him and are facing their own challenges from the gods who are reluctant to get involved.
Well, a cool plot could be that Percy is held captive in a magical prison created by a powerful sorcerer who has a vendetta against the gods. The prison suppresses Percy's powers, and he has to rely on his friends from outside to send him clues on how to break free. His friends, like Annabeth, might be on a quest to find the key to the prison which is hidden in a dangerous, enchanted forest filled with mythical creatures. As Percy waits, he discovers secrets about the sorcerer's past that could be his ticket out.
One plot idea could be that Percy is held captive by an ancient, forgotten sea deity who wants to use Percy's powers for its own dark purposes. Maybe this deity has the ability to manipulate water in ways that even Percy hasn't seen, and he has to find a way to outwit it using his wits and the limited power he can access while captive. Another idea is that he's held captive by a group of rogue demigods who believe Percy is a threat to their new world order. They try to brainwash him into joining their side.
There's a possibility that a new, unknown and powerful monster could be the captor. This monster could be a hybrid of different mythological creatures, created by some dark magic. It has heard of Percy's fame and power and wants to keep him captive either to study him, or to use his power to become even more powerful itself. It could have a lair in a hidden and dangerous place, like an underground cavern filled with deadly traps.
Well, it depends on the specific fanfiction. Sometimes, Percy might get help from an unexpected source. He could befriend a magical creature that is also held captive by Luke and together they come up with an escape plan. He might also discover a hidden passage or weakness in Luke's lair that he can use to his advantage. Another possibility is that his friends from Camp Half - Blood somehow find out where he is and stage a rescue mission.
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.