One popular trope is the 'enemies - to - lovers' scenario. Given their somewhat antagonistic relationship in the original story, it's a natural progression in fanfiction. Another is the 'forced proximity' trope, perhaps they are forced to work together on a mission, which then leads to the development of their relationship.
A less common but interesting trope is the 'role - reversal'. For example, Illumi being the more emotional and impulsive one in the relationship while Hisoka is the more composed and controlling. This flips the expected dynamic based on their original personalities and can lead to very creative and engaging storylines.
In Hisoka x Illumi yaoi fanfictions, the attraction between them could be based on mutual respect for each other's power. Hisoka is always seeking strong opponents, and Illumi is a very powerful assassin. Their interactions in the original work provide a rich source of material for fanfiction writers to develop romantic or at least deeply emotional relationships between them.
No, they aren't. There's no such plot or indication in the manga.
No, they aren't. There's no such plotline in the manga.
No, they aren't. There's no indication of them being married in the manga.
No, they aren't. There's no such plot or development in the manga.
I think Hisoka is stronger. His fighting skills and strategies seem more unpredictable and powerful in the manga.
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.
No, they didn't. There's no such plot in the manga.
The storyline usually involves the complex relationship and interactions between Hisoka and Illumi, with lots of tension and mystery.
The comic featuring Illumi and Hisoka usually depicts their battles of wits and skills. Their storylines are filled with unexpected turns and hidden agendas.