For this 'god of war atreus crossover fanfiction', characters like Beowulf would be a great crossover. Beowulf is a heroic figure in Anglo - Saxon literature, and his encounter with Atreus could be a clash of two different heroic cultures. Another one is Aragorn from The Lord of the Rings. Aragorn's leadership and Atreus' growth as a young hero could make for an engaging story. And don't forget about Dante from Devil May Cry. Dante's flamboyant fighting style and Atreus' more reserved Norse combat style could create a really fun dynamic in a crossover.
A possible plot for an Atreus and Percy fanfic could be centered around a prophecy. The prophecy states that only the combined strength of a Norse and a Greek hero can stop an ancient evil from awakening. Atreus and Percy are brought together by this prophecy. At first, they are skeptical of each other because of their different backgrounds and ways of doing things. However, as they start to face the challenges related to the prophecy, such as deciphering ancient texts and fighting off minions of the evil, they gradually learn to trust and rely on each other. In the end, they succeed in preventing the evil from rising, and their friendship becomes a legend in both their worlds.
Finally, focus on the interactions. How will Atreus interact with the characters in the new universe? If it's a crossover with the Marvel universe, how would he get along with Thor or Loki? His reactions and relationships with these new characters will drive the story forward. You can also add some action scenes that combine Atreus' skills with the unique elements of the new universe, like having him fight alongside the Avengers using his Norse - inspired combat moves.
Atreus's power of Norse magic and his proficiency with the bow could work in interesting ways with Percy's powers. Imagine a scene where they are facing a powerful enemy. Atreus uses his magic to create illusions to distract the foe, while Percy uses his water - based powers to create a smokescreen. Then, Atreus shoots his arrows through the smokescreen, guided by Percy's ability to sense the enemy's location through the water vapor. Their powers could also clash at times. For instance, if Percy accidentally creates a flood in an area where Atreus is trying to use his earth - based Norse magic, they would have to quickly figure out how to balance their powers.
It might be a story about his family, not just his own past but also about Atreus' mother. Kratos could be sharing memories of their life together, how they met, and the kind of person she was. This would give Atreus a deeper connection to his family heritage and a better understanding of his own identity.
The story Kratos told Atreus might have been about his journey to redemption and the lessons he learned along the way. Maybe it included details of the difficult choices he had to make and the consequences of his actions.