A great plot for Percy Jackson and Descendants fanfiction could be a quest. The characters from both universes have to team up to find a powerful item that has the ability to either save or destroy both their worlds. This would force them to overcome their differences and learn from each other. Maybe a character from Descendants has a special ability that the Percy Jackson crew needs to complete the quest, and vice versa. For instance, Mal's magic could be useful in certain situations that the demigods can't handle with their powers alone, and the demigods' combat skills could help the Descendants in battles.
Another plot could involve a character from Percy Jackson, like Annabeth, getting transported to the Descendants' world and having to adapt to their different rules and magic system. She could use her wisdom to help solve a mystery that has been plaguing the Descendants, such as the origin of a new dark magic that is starting to spread. This would lead to her making new friends and enemies in the process.
One idea could be that the demigods from Percy Jackson's world accidentally end up in the Descendants' universe. They could team up with the VKs (Villain Kids) to face a common threat that is related to both Greek mythology and the magic in the Descendants world. For example, an ancient Greek monster that has been awakened by the dark magic in the Isle of the Lost.
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.