Killua is definitely one of the most intriguing. His journey of breaking away from the family's assassin ways and his complex relationship with his siblings and parents offer a lot of material for fanfiction. His growth and self - discovery make him a great character to write about.
Love in the context of the Zoldyck family is an interesting theme. It could be love between family members that defies the cold - blooded assassin image. Or perhaps a love story that involves a Zoldyck member and an outsider, which challenges the family's traditions and values.
Killua is likely to be completely devastated. He might go into a state of shock at first, not believing that Gon is really gone. Then he could become very angry, either at himself for not being able to protect Gon or at the situation that led to Gon's death.
One popular theme is the exploration of the different Nen abilities in more detail. Fanfics might focus on a character developing a new Nen ability or learning more about the existing ones. Another theme could be about the various cultures within the Hunter x Hunter world, like the culture of the Phantom Troupe or the Zoldyck family. These can be really interesting as they give more depth to the already rich world of Hunter x Hunter.
The Hunter x Hunter manga is unfortunately not complete. The author has faced various issues that have led to intermittent breaks and an unresolved storyline.