A really interesting plot could be that Percy, along with his friends from Camp Half - Blood, are sent on a quest to Troy. There, they find that the demigods of that time had a different set of rules and powers. They have to navigate the political intrigue among the Trojans and the Greeks, all while trying not to disrupt the natural flow of history too much. But of course, things go wrong and they end up in the middle of a huge battle, using their unique demigod abilities to survive.
Another idea is that Percy discovers a connection between the Greek gods in his world and the events of Troy. He might find out that some of the monsters he has fought in his own adventures are related to the ones that were present during the Trojan War. For example, the Hydra could have been a creation of a Trojan - era sorcerer, and Percy has to stop it from being reborn.
One plot idea could be Percy Jackson somehow getting transported to the time of the Trojan War. He could use his demigod powers to influence the outcome. Maybe he teams up with Achilles, and together they face off against the gods who are meddling in the war in ways different from the original myth.
One plot idea could be that Percy Jackson is sent on a quest that somehow intersects with the story of Helen of Troy. Maybe he has to prevent a new Trojan War from starting. Another could be that Helen of Troy has some sort of hidden power that only Percy can unlock, and they have to team up to defeat a common enemy. Or perhaps Percy gets transported back in time to the era of Helen of Troy and has to navigate the complex politics and mythology of that time.
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.