Another theme is the idea of an unorthodox relationship. They are both rather strange characters in their own ways, and fanfics usually show how their relationship is not like a normal friendship or romance. It's something much more complex and full of intrigue.
The theme of loyalty also pops up a lot. Even though their relationship is so unusual, there are moments where they show a certain level of loyalty to each other, whether it's because of self - interest or some deeper, unspoken connection. In many fanfics, this loyalty is tested and explored in various scenarios.
There's 'The Unlikely Alliance'. In this fanfic, Hisoka and Illumi find themselves in a situation where they have to work together. It shows their initial distrust but also how they gradually come to understand each other's strengths and weaknesses, which is really interesting to read.
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.
Another theme is the psychological aspect. Hisoka is a very complex character. Fanfics usually explore how the self - inserted character deals with Hisoka's unpredictable and often disturbing psyche. It can be about fear, fascination, or both.
One common theme is the exploration of their opposing personalities. Hisoka is wild and unpredictable, while Machi is more reserved and practical. Fanfictions often play with how these differences can create either conflict or an unlikely attraction.
A prevalent theme is also the idea of an unlikely alliance. The OC might form an alliance with Hisoka for various reasons, like a common enemy or a shared goal. And of course, there's the theme of self - discovery. As the OC interacts with Hisoka and other characters from 'Hunter x Hunter', they often learn more about themselves and their own capabilities. For example, in some fanfictions, the OC discovers hidden powers while facing challenges related to Hisoka.
One plot idea could be that Illumi and Hisoka are forced to team up on a mission. Their different personalities - Illumi's cold and calculating nature and Hisoka's flamboyant and unpredictable one - would create a lot of tension and interesting interactions. Maybe they have to infiltrate an enemy stronghold and they constantly bicker about the best way to do it.
There's a fanfic called 'Hisoka's Obsession with Machi'. It delves into Hisoka's perspective. He is constantly intrigued by Machi's skills and her cold exterior. The story has some interesting scenes where Hisoka tries to get closer to Machi, and Machi is both repelled and fascinated by him at the same time. It's a really engaging read.
A common theme is the conflict between their different paths. Killua wants freedom and to make his own choices, while Illumi is bound by the family's rules and expectations. Fanfictions often explore how they deal with this divide.