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.
Yes, there could be some. Some fanfiction might focus on a more innocent or platonic relationship within the context of Illumi and a pregnant character, perhaps exploring themes like friendship or family support rather than anything inappropriate.
In many 'HxH Illumi x Reader' stories, the reader has a special power that Illumi wants to use for his own purposes. However, as he spends more time with the reader, his feelings start to change from just using them to actually caring about them. This kind of story often shows the complex character of Illumi as he struggles between his cold - hearted nature and his new - found emotions.
One great 'HxH Illumi x Reader' story could be about the reader being a new hunter and Illumi taking an unexpected interest in them. Maybe he's intrigued by their unique nen ability or their fearless attitude towards the dangerous tasks hunters face.
Friendship or brotherhood is also a big theme. Despite their differences, many fanfictions show how they support each other in their own ways, like Killua relying on Illumi's skills in certain dangerous situations and Illumi having a soft spot for Killua deep down.