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.
Kazuma is often portrayed as a bit of a schemer. He's not overly powerful in terms of magic but is very clever. He can come up with quick solutions to problems that rely more on his wits than brute force. In the context of a fanfic with Yunyun, he might be the one to keep Yunyun grounded and out of trouble, or sometimes get her into new and interesting situations.