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.
One interesting Kratos and Atreus fanfiction could be about their adventures in a new, uncharted realm. They might encounter strange creatures and have to use their combined skills to survive. For example, a story where they come across a land filled with ice giants and have to figure out how to defeat them without freezing to death.
One common theme is the re - imagining of the mentor - apprentice relationship. Since Kratos is often seen as a mentor to Atreus in the original, when Kratos is female, it can bring a different flavor to that dynamic. For example, a female Kratos might be more empathetic in her teaching methods.
One common theme is redemption. Both Kratos and Freya have done things in their pasts that they regret, and fanfics often explore their paths to redemption. Another theme is friendship. Despite their differences, they can form a strong bond. For example, in many fanfics, they start to trust each other and rely on one another in difficult situations.
One common theme is growth. Both Kratos and Atreus are constantly evolving in their abilities and understanding of the world around them. Fanfictions often explore how they grow individually and as a team.