One idea could be a quest where Magnus and Percy have to team up to retrieve a powerful artifact that is causing chaos in both their worlds. Maybe it's an ancient Norse and Greek combined relic that only their combined skills can handle.
A great story could involve the two of them getting accidentally transported to a strange dimension where the rules of both Norse and Greek mythology are all jumbled up. They must work together to find their way back while also dealing with the mixed - up mythical creatures. For example, Percy might face a draugr that has the powers of a Greek hydra. They would need to figure out how to defeat it using their different knowledge and abilities. Percy's water - based powers and Magnus's ability to talk to the dead could be combined in interesting ways to solve problems in this new dimension.
Another idea is a setting where the two worlds are merging. Percy and Magnus find themselves in a strange new place that has elements of both Camp Half - Blood and the Hotel Valhalla. They have to figure out how to survive in this new world while also dealing with the chaos that the merging is causing. The characters from both series could interact in interesting ways, like Annabeth and Samirah comparing their knowledge of architecture and magic respectively.
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.
The Titans from Greek mythology could also be a good choice. Imagine if the Titans, who are still bitter about their defeat by the Olympians, team up with some of the more malevolent Norse giants. They could try to take over both the Greek and Norse realms, forcing Percy and Magnus to join forces to stop them.
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.
Percy's water - based powers and Magnus's ability to communicate with the dead could create some really interesting scenarios. For instance, Percy could use water to expose hidden passages in cemeteries where Magnus could then communicate with the spirits to get information.
Time - travel is another trope that shows up. Writers might send Percy or Magnus back in time to change something in the other's world. It creates a lot of opportunities for adventure and exploring different time periods in the Riordanverse. This can also lead to some really cool crossovers between the mythology of the two series.
One interesting crossover could be a quest where Magnus and Percy have to team up. They could combine their different powers, like Percy's water - based abilities and Magnus's connection to Norse mythology. For example, they might face a sea monster that has a connection to both Greek and Norse realms. Another could be a story where they end up in each other's worlds and have to adapt. Percy in the Norse world would be really fun to read as he tries to figure out the different rules and gods there, and vice versa for Magnus in the Greek world.
Another common theme is character relationships. This could be romantic relationships, like a pairing between a Magnus Chase character and a Percy Jackson character. Or it could be friendships, exploring how characters from the two different series would be friends.
One interesting crossover could be the characters from both series teaming up to fight a common enemy. For example, Magnus and Percy joining forces against a powerful ancient monster that threatens both the Norse and Greek worlds. This would be exciting as they both have different powers and abilities that could complement each other.
First, do your research on both series. Know the characters, their abilities, and the rules of their respective worlds. For example, Percy can control water because he's Poseidon's son, and Magnus has the power of Sumarbrander, his magical sword. When you write their interactions, make sure it's consistent with their personalities. Percy is often impulsive and loyal, while Magnus is more laid - back and has a sarcastic sense of humor. Make sure their dialogue reflects this.