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.
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.
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.
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.
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.
If Percy reads positive fanfiction about Magnus, he might develop a new - found respect for him. He could start to think of Magnus as more of an equal in the world of demigods, even though they are from different mythologies. For instance, if the fanfiction shows Magnus doing heroic deeds, Percy would recognize the similarities between their roles as heroes. However, if the fanfiction portrays Magnus in a negative way, Percy might be a bit more cautious around him, wondering if there's some truth to those made - up aspects.
Well, it's really hard to say. Percy has his water powers and his combat skills which are no joke. But Magnus has his own set of powers and his connection to Norse mythology is super strong. If it's a battle in water, Percy might have the upper hand. However, if it's in a more Norse - related environment like a frozen tundra or near a great hall in Asgard, Magnus could use the elements of his world to his advantage.
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.
I really like the idea of Percy and Magnus just chatting about their lives. Percy talking about his experiences with the Greek gods and Magnus sharing his Norse - related adventures. It would be a simple but really engaging interaction.
Loki could be a great villain. He's known for his trickery in Norse mythology. He could manipulate events to set Magnus and Percy against each other. Maybe he wants to cause chaos between the Norse and Greek worlds for his own amusement or to gain more power.