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.
I don't think so. There haven't been any official announcements or indications that the HxH manga will continue. But you never know, maybe there'll be a surprise in the future.