One interesting fanfiction could be about the reactions of the human side when a new and unexpected god shows up to fight. For example, how they strategize and what their initial thoughts are.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, including Record of Ragnarok. Just search for'record of ragnarok reaction' and you'll likely find some good ones.
One common theme is the idea of the reader being an outsider in the world of Record of Ragnarok. They often have to adapt to the strange and dangerous environment. Another theme is the power struggle, as the reader may get involved in the battles between gods and humans.