Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
Another common theme is adventure. Fans might write about josuke and okuyasu going on new, exciting adventures together that are related to their Stand abilities. It could be them facing new enemies or exploring strange places. Also, there can be a lot of stories that focus on their personal growth, like how they become better versions of themselves with the help of each other. These fanfics might show okuyasu becoming more confident or josuke learning to be more patient, all because of their relationship.
Yes, there are quite a few. You can start by looking on popular fanfiction websites like Archive of Our Own. There, you'll find a variety of stories exploring the relationship between Josuke and Okuyasu, from friendship - focused ones to more romantic interpretations.
One common theme is friendship. Their strong bond in the original story is often explored more deeply. Another is loyalty, showing how they would always have each other's backs. And sometimes, there's a bit of a coming - of - age theme as they grow and develop their relationship.