One interesting Noragami fan fiction could be about Yato's past adventures before he met Hiyori. It might explore his relationships with other minor gods and the challenges he faced. Another could be a story that focuses on the friendship between Yukine and Hiyori, and how they support each other when Yato is in trouble. There are also fan fictions that re - imagine the whole Noragami world with different rules and powers for the characters.
You can start by looking on fan fiction websites like Archive of Our Own (AO3) or FanFiction.net. Just search for 'Noragami' in their search bars. AO3 usually has a good tagging system, so you can filter by things like genre, rating, and length to find the fan fictions that suit your taste.
In Noragami fanfics, Yato and Yukine's bond is often portrayed in unique ways. One way is through the exploration of their spiritual connection. Since Yukine is Yato's weapon and also his partner in a sense, fanfics can play around with how this connection affects their actions and decisions. For instance, a fanfic might show how Yato can sense Yukine's emotions through their bond when they are in a dangerous situation.
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.