Love and loyalty are themes that are often explored as well. It's not always romantic love, but a deep, unwavering loyalty that might exist between them. Some fanfictions show how, despite their differences, they would still be there for each other in difficult situations, like when facing a common enemy or dealing with the challenges within their family.
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.
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.
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, a very frequent theme is the exploration of power dynamics. Given Illumi's powerful and somewhat intimidating nature, the fanfictions often look at how the OC fits into that power structure. It could be that the OC is weak at first and has to grow to be on par with Illumi, or the OC might have a unique power that changes the balance. Also, love and redemption are common themes. The OC might bring out a sense of redemption in Illumi, or they might fall in love in a rather complex and interesting way considering Illumi's character.
There's 'Illumi's Obsession'. In this fanfic, it delves deep into how Illumi's feelings for Killua are complex and often misunderstood. It shows how their interactions can be both intense and full of unspoken emotions. It's quite engaging as it plays with the psychological aspects of their relationship.
Friendship is a very common theme. Since Gon and Killua have such a strong bond in the original work, fanfictions often explore different aspects of their friendship, like loyalty and mutual understanding.
A common theme is family. Given their complex family situation in the original story, fanfictions often explore how Killua and Alluka's relationship affects and is affected by their family. Another theme is protection. Killua is very protective of Alluka, and this is often a central aspect in these fanfictions.
One common theme is self - discovery. The OC often discovers new things about themselves while being with Killua. Another theme is friendship evolving into something more. For example, they start as friends or allies and then gradually develop romantic feelings. Also, there's the theme of overcoming differences. Since the OC is not from the original story, they usually have different values or abilities than Killua, and they have to overcome these differences to be together.
One common theme is the exploration of power dynamics. Hisoka is much more powerful than Gon and Killua at first, so fanfics often play with how they either try to overcome this gap or how Hisoka uses his power in relation to them.
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.