Over time, their relationship could become more complex. Illumi's loyalty to his family and Hisoka's self - centered goals might clash. But they could also find common ground in their pursuit of power. They might start training together, which would lead to a deeper understanding of each other. Hisoka could learn about Illumi's cold family upbringing and Illumi could understand Hisoka's obsession with fighting strong opponents better.
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.
At first, they might be very close due to their shared origin as Chimera Ants. But as they experience different things, their relationship could become strained. For instance, if the OC brother starts to question the Chimera Ants' violent ways and Pitou remains loyal to the King, there would be conflict. However, deep down they still care for each other, so they might try to find a middle ground.
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 '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.
One interesting fanfiction could be about Killua and Illumi's relationship evolving. Maybe Killua is trying to break free from Illumi's over - protective and controlling ways. They could end up having a heart - to - heart conversation where Killua makes Illumi understand that he needs to grow on his own terms.
Well, Hisoka's origin story in Hunter x Hunter is quite interesting. He hails from Meteor City, a place that's more like a dumping ground for the unwanted. Growing up there, he was exposed to all sorts of things that normal people wouldn't experience. His love for fighting strong opponents probably stems from the fact that in Meteor City, strength was the only way to get what you want. Hisoka's nen abilities are also a product of his environment. He's constantly evolving and seeking new challenges, which is likely a trait he developed while growing up in a place where only the fittest survive.
Her character could start off as innocent, unaware of her father's dark side. As she grows and experiences more of the world of Hunters, she becomes more complex. For instance, she might witness some of Hisoka's cruel acts and be conflicted between her love for him as a father and her moral compass.
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.
I really like 'Hisoka's Obsession'. It shows Hisoka's fixation on Gon in a more detailed and somewhat darker way. It adds layers to Hisoka's character while also developing Gon's reactions to Hisoka's strange behavior. The writing style is engaging and it's easy to get lost in this alternative take on their relationship.
To develop their relationship realistically, start with small interactions. Feitan is a reserved and dangerous character. So, the OC should be cautious around him at first. Maybe they start with just a glance or a short exchange of words. As time goes on, the OC could show Feitan kindness in a way that he's not used to, like sharing food. This would slowly break down his walls. Also, the OC should have her own goals and personality that sometimes clash with Feitan's, which would add depth to their relationship.