One possible plot could be Percy Jackson being magically transported to the world of the Sons of Sigmar. He could discover that his demigod powers have a different manifestation there. Maybe he teams up with a Son of Sigmar to fight a common enemy that threatens both their worlds, like a powerful dark sorcerer who is trying to open portals between the two realms to bring chaos.
Perhaps a powerful daemon from the underworld who sees the connection between Percy's world and the world of the Sons of Sigmar as an opportunity. This daemon wants to use the chaos that would result from a clash between the two worlds to break free from its own realm and dominate all of existence. It manipulates both sides, creating mistrust and starting battles between them.
Maybe in the fanfiction, the zoo is a place where the gods have hidden some of their lost magical artifacts. Percy, being the son of Poseidon, senses a strange water - based magic near the aquarium section. He starts to investigate with Annabeth. Along the way, they face challenges from zoo security (who are unknowingly being influenced by a minor god who doesn't want the artifacts found) and various magical traps set around the exhibits.
A plot could involve Percy being sent to Forks on a quest. He discovers that there's a powerful artifact hidden in the area that could cause chaos if it falls into the wrong hands. The vampires and werewolves in Forks are also after this artifact for their own reasons. Percy has to navigate the complex relationships between these two groups while also trying to keep his true identity as a demigod a secret. Along the way, he forms an unlikely friendship with Jacob, who helps him understand the ways of the werewolves.
One possible plot could be Percy arriving in the DxD world and being mistaken for a new, extremely powerful devil. He then has to navigate the complex politics of the devils while also trying to find a way back home. Along the way, he uses his demigod powers in unique ways against the various threats in the DxD universe.
One possible plot could be that the Volturi from Twilight discover the existence of demigods from the Percy Jackson world. They see the demigods as a threat to their vampire supremacy. Percy and his friends have to team up with some of the more friendly vampires, like the Cullens, to fend off the Volturi. This would bring together the magic and powers of both universes in an epic battle.
Well, perhaps there's a villain in the Percy Jackson universe who discovers that the iCarly web show has a huge following and decides to use it as a way to spread chaos. The demigods have to team up with the iCarly crew to stop this. The iCarly gang has to learn about the world of Greek mythology really fast and help the demigods navigate the modern world of technology and media to defeat the villain.
Maybe Percy is on a quest and stumbles upon a group of magical creatures playing poker. They invite him to join. Percy, being Percy, is a bit clumsy at first but then realizes that the game has a deeper meaning related to the prophecy he is part of. He starts using his battle - trained mind to analyze the opponents' moves and the patterns of the cards, all while trying not to reveal his true identity as a demigod.
Well, it could start with a magical accident. In the Percy Jackson universe, a spell gone wrong sends Percy, Annabeth, and Grover into the land of Alagaësia from Eragon. There, they discover that the demigod powers Percy has are seen as a strange and new form of magic. Eragon is initially suspicious of them but realizes they can help in his fight against Galbatorix. They combine their skills, Percy's water - controlling powers and sword - fighting skills, along with Annabeth's strategic mind and Grover's nature - sensing abilities, with Eragon's dragon - riding and magic - wielding skills to take on the dark forces.
The plot could involve a powerful magical artifact that has military applications. The NCIS comes across this artifact during an investigation but doesn't understand its true nature. Percy Jackson and his friends, who are on a quest to retrieve the artifact for the demigod world, cross paths with the NCIS. There is a lot of miscommunication at first as the two groups have different goals and ways of operating. But as they realize that they are both facing a common threat from a group of rogue demigods and power - hungry military personnel, they join forces. Percy uses his knowledge of the demigod world and his powers to help the NCIS agents navigate the magical aspects while the agents use their resources and combat training to take on the human threats.
One possible plot could be Percy waking up as a girl and having to deal with the new physical and social aspects in Camp Half - Blood. Maybe the other campers are initially confused, but then they see that Percy (now a girl) still has the same heroic spirit and skills. She could go on a quest where her new female form gives her a different perspective on things, like how she interacts with female monsters or how she is treated by male gods differently.