Well, Hisoka's origin story in Hunter x Hunter is quite interesting. He hails from Meteor City, a place that's more like a dumping ground for the unwanted. Growing up there, he was exposed to all sorts of things that normal people wouldn't experience. His love for fighting strong opponents probably stems from the fact that in Meteor City, strength was the only way to get what you want. Hisoka's nen abilities are also a product of his environment. He's constantly evolving and seeking new challenges, which is likely a trait he developed while growing up in a place where only the fittest survive.
One common theme in fanfics about Killua's sister is her growth and development. Writers may show her learning more about the world, becoming more independent, and perhaps even using her powers for good. She might go on adventures, similar to the main characters in Hunter x Hunter, and meet new and exciting people along the way.
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.
The Hunter x Hunter manga has indeed concluded. However, the ending might have left some fans wanting more or with mixed feelings depending on their expectations.