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.
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.
Another theme is power. Given that both are very powerful in their own ways, fanfics often deal with how they compare their powers, use their powers in relation to each other, or how their powers influence their relationship.
The theme of loyalty also pops up. In some fanfics, they test each other's loyalty in various situations. It could be in the face of danger or when their own self - interests are at stake. This adds an interesting layer to their relationship as they navigate through these challenges together.
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.