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.
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.