The story of 'Hunter x Hunter' follows Gon Freecss, a young boy who discovers that his father, whom he thought was dead, is actually a world - renowned Hunter. Gon decides to become a Hunter himself to find his father. Along the way, he meets other Hunters - in - training like Killua Zoldyck. They face various challenges, from the deadly Hunter Exam to battles against powerful foes like the Chimera Ants. The story is filled with complex characters, intense battles, and a unique power system.
One way to incorporate an OC into the plot of a Hunter x Hunter fanfic is to make them a part of an existing story arc. For example, if the fanfic is set during the Chimera Ant arc, the OC could be a new Hunter sent to help fight the Chimera Ants. They could have a special skill or knowledge that is crucial in the battle. Another way is to create a new story arc centered around the OC. Maybe they discover a hidden danger in the Hunter x Hunter world that the other characters are unaware of, and they have to convince Gon, Killua, and the others to join them in facing this threat.
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.
One possible plot could be that Killua decides to surprise Gon with breakfast in bed. He gets up early, sneaks into the kitchen, and tries to cook Gon's favorite meal. However, he's not very good at cooking, so there are a lot of comical mishaps. In the end, Gon wakes up to a messy kitchen but a very sweet gesture from Killua.
The Hunter x Hunter manga is unfortunately not complete. The author has faced various issues that have led to intermittent breaks and an unresolved storyline.