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 a crossover where Percy and Magnus have to team up to defeat a common enemy that threatens both their worlds. Maybe a primordial being that can disrupt the balance of both the Greek and Norse mythologies.
A really cool plot could be that there's a prophecy that involves both of them. The prophecy says that only by combining their unique abilities - Percy's control over water and Magnus's connection to the Norse gods - can they save both the Greek and Norse mythologies from being wiped out by a new, unknown force. They have to go on a journey, meet various characters from both mythologies, and overcome many challenges along the way.
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 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.
One idea could be a quest where Magnus and Percy have to team up. Maybe there's a new threat that combines the Norse and Greek mythologies, like a monster that has the powers of both a Greek hydra and a Norse dragon. They would have to use their different skills and knowledge to defeat it.
One idea could be that Percy's parents are from different worlds, like his mom is from a modern human world and his dad is from a magical realm. When they meet, there are cultural clashes and misunderstandings. For example, his dad doesn't understand modern technology and his mom is shocked by the magical creatures his dad brings along. Another idea is that Percy has been hiding his true self from his parents, and when they finally meet, he has to come clean about his adventures and powers. This could lead to a range of emotions from disbelief to pride. Or perhaps there's some sort of danger that forces Percy and his parents to meet and work together, and in the process, they really get to know each other for the first time.
One idea could be that Magnus and Clary discover the pregnancy unexpectedly. They then have to deal with the reactions from their friends and family. Some might be overjoyed while others could be worried about the challenges ahead.
One plot idea could be Magnus trying to balance his powerful magic with the changes in his body during pregnancy. He might be worried about how his magic could affect the unborn child. Another could be the reactions of his friends and family. For example, Alec might be over - protective and trying to keep Magnus safe from all the possible threats in the Shadowhunter world. And Magnus could also be facing challenges from the Downworlders who might see his pregnancy as a sign of weakness or a change in the power dynamics.
There could also be a plot where Magnus and Max are from different dimensions or time periods and accidentally meet. At first, they don't understand each other because of the differences in their worlds. But as they start to communicate and learn about each other's cultures and powers, they find common ground. They might have to work together to find a way to get Max back to his own time or dimension, and in the process, they create memories and a relationship that changes both of them for the better.
One idea could be the discovery of new powers within the Staff of Magnus. Maybe it has the ability to open portals to different magical realms that were previously unknown in the Harry Potter universe. This could lead to adventures where the characters explore these new places and encounter unique magical creatures.