The theme of power and growth is quite prevalent too. Writers often depict how they grow in terms of their nen abilities and how their relationship evolves as they become more powerful. For example, in some fanfictions, they train together and through that process, their bond becomes stronger while they also gain new powers.
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.
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.
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.
One popular illumi and killua fanfiction is 'A Brother's Bond'. It delves deep into their complex relationship, showing how they interact beyond what's shown in the original work. It has some really interesting character development for both of them.
One common theme is power - up. When Killua crosses over into another universe, he often gains new abilities or finds ways to enhance his existing ones. For example, in a crossover with a superhero universe, he might learn to control a new type of energy. Another theme is friendship. He forms new bonds with characters from the other universe.
Another theme could be the relationship between Killua and the people around him, like his friends or family. For example, how Gon reacts to Killua's pregnancy and how they support each other. It could also touch on themes of love and sacrifice as Killua navigates this new situation.
Adventure is also a common theme. The characters from Killua's world and RWBY's world going on quests together. They might be on a mission to stop a powerful enemy that threatens both their universes. This usually involves a lot of action scenes, battles against unique and powerful foes. And of course, there's often some element of mystery, like trying to figure out the secrets of the other world or the true motives of certain characters in the crossover.
Another theme is friendship. Killua is used to a certain type of friendship from his own story, and when he meets RWBY characters, new bonds are formed. It's about how he adapts to their group dynamic and vice versa. For example, his relationship with Ruby, who is very different from his friends back home.
Adventure is also very common. The OC usually gets involved in Killua's dangerous and exciting world of being a Hunter. They might be on a mission to find a rare item or defeat a powerful enemy. And there's often a theme of trust - building. Killua has to learn to trust the OC and vice versa.
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.