Hisoka is still alive in the manga. The storylines haven't led to his death yet. Maybe there'll be developments in the future, but for now, he's kicking.
Definitely not. Hisoka and Illumi have a complex relationship in the manga, but marriage isn't part of it. Their interactions are focused on other aspects of the story.
One key element is his abnormal love for combat. He has always been drawn to fighting, even from a young age. Another is his lack of normal moral compass. This is evident as he does things just for his own pleasure in fighting.
Hisoka is a very interesting character. His origin story shows that he has always been a bit of an enigma. He was born with a rather twisted sense of pleasure, often finding joy in battles and testing his strength against strong opponents. From a young age, he was different from the norm, having a fixation on fighting and the thrill it brought him.
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.