I really like 'Kane Chronicles and Magnus Chase: A Crossover'. This fanfic is amazing as it brings together two Rick Riordan universes. It delves deep into how the different mythologies can co - exist and interact. The author does a wonderful job of creating new adventures for the characters from both series.
First, know both universes well. For example, understand the magic system in the Kane Chronicles and the character traits of its main characters. Then, create a believable reason for the crossover. Maybe a magical portal opens between the two worlds. Also, keep the voices of the characters true to their original portrayals.
Anubis would be a great character to include. His complex relationship with Sadie in the original series could be further explored in a crossover. He could bring his knowledge of the underworld and death magic to the story. Also, if it's a crossover with a different mythology, say Greek mythology, Hermes could be included. Hermes' speed and trickery could contrast well with the more structured magic of the Kanes, and there could be some interesting interactions between them.
A cool plot could involve a crossover with a modern - day superhero universe. The Kanes use their magic to help the superheroes defeat a villain who has somehow harnessed ancient Egyptian magic. They could teach the superheroes about the different aspects of Egyptian magic and how it can be used against the villain. For example, using the power of Ra to counter the villain's dark spells.
One of the great ones is 'The New Adventures in the Kane Chronicles'. It takes the characters on new and exciting quests that feel like they could be part of the original series. Another good one is 'Kane Chronicles: Alternate Endings' which explores different ways the story could have ended. And 'The Kane Chronicles: Extended Tales' gives more backstory to the characters.
Maybe there's a magical artifact that has significance in both Egyptian and Greek mythology. The characters like Percy and Carter Kane have to team up. Percy could use his water powers and Carter his magic to navigate through various challenges. They might start off distrusting each other because of their different mythological backgrounds but then realize they need to work together to stop the artifact from falling into the wrong hands.
One idea could be that Kane discovers a portal to the Naruto universe and has to learn ninjutsu to survive. He could team up with Naruto and his friends to fight a common enemy like Orochimaru who is interested in Kane's unique powers from his own world.
Fanfiction can enhance the reading experience in multiple ways. First, it can fill in the gaps that the original books might have left. Maybe there was a side character that you were curious about, and fanfiction can give them a whole new story. Second, it allows for different interpretations of the events in the books. Different fanfic authors might see a scene in a different light and write a story based on that. It also keeps the fandom alive and engaged, making you think more about the world and characters of The Kane Chronicles even after you've finished the books.
A cool plot could involve a prophecy that links the two universes. Percy and Carter Kane could be the key figures. Maybe there's an ancient evil that once threatened both worlds long ago and is now rising again. The two heroes, along with their friends, have to figure out how to use the powers and knowledge from both their worlds to defeat this evil. They might start by sharing their combat techniques, like how the demigods fight with their weapons and how the magicians use their magic spells.
Naruto and Carter Kane would be an interesting pairing. Naruto's exuberant and determined personality combined with Carter's knowledge of magic and leadership skills could lead to some great adventures. They could learn from each other, like Naruto teaching Carter about chakra and Carter teaching Naruto about hieroglyphic magic.