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.
The theme of redemption can also be found. Sometimes, the fanfic will have Chrollo showing signs of wanting to change or make amends, especially in relation to Kurapika. And vice versa, Kurapika might start to question his single - minded pursuit of revenge in the face of Chrollo's complex character. Another common theme is the power dynamic between them. Given their different abilities and standings in the world of 'Hunter x Hunter', how they assert and negotiate power in their relationship is often explored in these fanfics.
One common theme is loneliness. Chrollo often seems isolated in the original story, and fanfics tend to explore how his past experiences, like losing his family or being an outcast, led to this loneliness. Another theme is power - seeking. Fanfics show how his past traumas made him strive for power as a means of survival and control.
Another theme is the exploration of their unspoken connection. They have a unique relationship in the original work, and in fanfiction, this is often expanded upon. It could be a connection based on mutual understanding of the harsh world they live in or their shared experiences in the Phantom Troupe. Some stories also play with the idea of a possible romantic undertone, although it's not always the main focus.