One interesting crossover could be a story where Naruto's world is invaded by ayakashi from the Noragami universe. Naruto and his friends have to team up with Bishamon's shinki. They would learn about each other's powers and cultures. For example, Naruto's ninjutsu might be a mystery to the Noragami characters, while the concept of shinki could be new to Naruto.
Naruto has the power of the Nine - Tails, which is a huge source of chakra. Bishamon has her own reservoir of divine power. In a fanfiction, they could combine their powers by having Naruto transfer some of his chakra to Bishamon, who then uses it to supercharge her attacks. Or vice versa, Bishamon could bestow some of her divine essence on Naruto, allowing him to access new forms or abilities that are a blend of both ninja and divine powers.
One plot idea could be Kazuma and Bishamon teaming up for a mission that no one else believes they can accomplish. They start off distrusting each other but as they face challenges, they gradually learn to rely on one another's strengths. Another could be a story where Kazuma discovers a long - hidden secret about Bishamon's past that changes their relationship dynamic.
Their relationship can be developed through shared experiences. For example, they could go through a tough battle together. During the battle, Kazuma could protect Bishamon from a near - fatal blow, making her see him in a new light. And Kazuma could also admire Bishamon's strength even more.
Sure, the Noragami manga is completed. It was a great ride with an ending that wrapped up the plot nicely. If you haven't read it yet, it's worth giving a go.