One popular theme is their friendship. Their bond is very strong in the original work, and fanfics often expand on it, like how they rely on each other in dangerous situations. Another theme is adventure. Since they are both on a journey in the original story, fanfics may create new adventures for them, such as exploring uncharted lands.
The theme of growth is also prevalent. They both start as somewhat naive boys and grow into powerful and more mature individuals. In 'naruto is like killua fanfiction', you might see how they influence each other's growth, for example, Naruto's positive outlook might affect Killua's more cynical view at times, and vice versa.
One popular 'killua and gon fanfiction' is 'A New Adventure'. It takes the two characters on a journey to a mysterious island not shown in the original series. Another great one is 'Bond Strengthened', which focuses on how their friendship deepens during a difficult quest. There's also 'The Hidden Powers' that explores new abilities they might discover.
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 great 'killua crossover fanfic' is 'Killua in the World of Naruto'. It combines the unique abilities of Killua from 'Hunter x Hunter' with the ninja world of Naruto. Another is 'Killua Meets the Avengers', which creates an interesting scenario where Killua interacts with the superheroes. There's also 'Killua's Adventure in Middle - earth' that blends the character into the Lord of the Rings universe.