A really cool plot for Percy Jackson fanfiction would be if Percy discovers a long - lost relative who is actually a powerful being from another realm. This relative has been banished and needs Percy's help to get back home. But in the process, Percy uncovers a dark secret about his family's past. He has to decide whether to help this relative or not, all while dealing with the normal chaos at Camp Half - Blood and threats from the monsters that are always lurking.
One idea could be Percy having to face a new and powerful underwater monster that is immune to his usual water - based powers. He'd have to rely on his wits and the help of his friends in unexpected ways. Another could be a time - travel story where Percy goes back to the era of the first demigods and has to learn from them to solve a modern - day problem. Maybe he teams up with an ancient hero and they combine their powers.
One idea could be that Percy is banished because of a false accusation of using his powers for evil. He then has to journey through unknown territories, facing new monsters and making unexpected allies to prove his innocence. Maybe he meets an ancient and forgotten god who offers him a deal to regain his status.
One idea could be Percy facing a new and powerful enemy that is immune to water, which forces him to develop new skills. Another could be Percy becoming a king in a new underwater realm and having to deal with political intrigue among the sea creatures. Maybe he has to unite different mythological groups under his leadership to face a common threat like an ancient primordial being awakening.
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.