One idea could be that they team up to face a new and powerful monster that neither of them could defeat alone. Maybe it's a hybrid of a Greek and Roman mythical beast.
Another plot could involve them getting into an argument over which of their respective camps has the better training methods, but then realizing they can learn from each other and work together more effectively. This could lead to a series of challenges where they combine their skills and knowledge to overcome obstacles. For example, Percy using his water - based powers and Jason using his lightning powers in a coordinated attack against an enemy that has defenses against both elements.
One idea could be a time - travel plot where Percy and Jason are sent back to the founding of Camp Half - Blood. They have to work together to ensure the camp is established correctly while also dealing with the ancient monsters and gods that were around at that time. Another could be a story where they swap powers for a day and have to learn to adapt to each other's abilities in the middle of a big battle against a new powerful enemy. A third idea is that they are stranded on a mysterious island together and must rely on each other's skills and knowledge to survive and find a way back home.
One idea could be a quest where Percy, Jason and Piper have to team up to find a lost artifact that has the power to control the elements. They face various mythical creatures along the way and have to use their combined powers to overcome obstacles.
One plot idea could be that Percy and Jason discover they are twins in a very unexpected way. Maybe during a quest, they start having the same strange dreams and realize they have a deeper connection. Another could be that they have to team up against a powerful enemy who is specifically targeting them because of their unique twin bond.
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 that Percy and Jason have to team up to save a group of demigods who are trapped in an ancient, cursed maze. They use their combined powers of water and lightning to break through the various obstacles. Another could be a story where they swap powers for a day and have to deal with the consequences. For example, Percy tries to control lightning but accidentally zaps some of their friends' hair into crazy styles, while Jason struggles to keep the water from going wild when he tries to use Percy's power.
A really cool plot could be that there's a prophecy that involves both Percy and Magnus. They initially don't trust each other but as they start to work together to fulfill the prophecy, they realize that they have more in common than they thought. Along the way, they might meet other characters from both of their stories, and there could be some epic battles and heart - warming friendships formed. There could also be a love interest involved, perhaps a character from one world falls for a character from the other world, adding a bit of romance to the mix.
One idea could be that Percy meets Amphitrite while on a quest in the ocean. She tests him to see if he's worthy of her son, Poseidon. Another could be that Amphitrite mistakes Percy for a threat at first but then realizes he has a pure heart. Maybe Percy could be seeking her help to save the ocean from a new and dangerous threat.
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.