The plot could also involve her having unique nen abilities different from Hisoka's. She might be on a journey to understand these powers and in the process, cross paths with the main characters from Hunter x Hunter. Perhaps she forms an unlikely alliance with Gon and Killua, who initially are wary of her because of her relation to Hisoka, but later come to respect her for her own qualities.
Another idea is that Hisoka's daughter is raised in a completely different environment from her father. However, she has a natural inclination towards the same kind of chaos that Hisoka loves. She could enter the Hunter Exam not knowing who her father is, and as she progresses, she starts to learn about her heritage and has to deal with the shock and the new - found knowledge while also competing against other strong candidates.
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.
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.
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.
One popular Hisoka fanfiction might be those that explore his backstory. Since in the original 'Hunter x Hunter' his past is quite mysterious, fanfictions often fill in the gaps. For example, there could be stories about his early days as a Hunter and how he developed his unique nen abilities.
A theme that often shows up is Hisoka's relationships, especially his strange dynamic with Gon and Killua. Some fanfictions focus on how Hisoka views these young hunters and how his interactions with them shape the story. Also, his flamboyant and unpredictable nature is a theme that writers like to play with, creating all sorts of scenarios based on his unique personality.