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.
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.
In Hisoka x OC fanfiction, the OC often serves as a way to insert new perspectives into the story. The author can give the OC special abilities or a unique personality that either complements or clashes with Hisoka. If the OC has a strong sense of justice, for instance, it would be in direct contrast to Hisoka's more self - serving nature. This contrast can drive the plot forward, creating conflict and resolution. Also, the development of their relationship can be a major focus. It could start from a simple encounter and gradually evolve into a complex bond, whether it's a friendship, a rivalry, or something more romantic. The key is to make the relationship believable within the context of the original story's universe.
One great Hisoka x Reader fanfiction is 'Hisoka's Obsession'. It delves deep into the complex relationship between Hisoka and the reader, with a lot of psychological exploration. Another one is 'A Dance with Hisoka' which has a really interesting plot where the reader character is involved in Hisoka's dangerous world of battles and power plays. And 'Hisoka's Secret Admirer' is also a good read as it shows a more tender side of Hisoka towards the reader in a very unique way.