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.
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.
A well - written Gon and Killua fanfic often has a strong focus on their friendship. It shows how they complement each other, like Gon's impulsiveness and Killua's strategic thinking. Good character development is key. For example, showing Killua becoming more open and Gon becoming more aware of the darker side of the world.
Sure, many fanfic writers create works that celebrate the bond between Gon and Killua in a proper way. You could start by looking at fanfic communities where moderators ensure the content is appropriate. Some fanfics may be about their first meeting and how that led to a deep and lasting connection. Others could be set in alternative universes where they still have that special friendship, and these are all great alternatives to the inappropriate ones.
One interesting Killua in My Hero Academia fanfiction is 'Killua in the World of Heroes'. It has an exciting plot where Killua's assassin skills blend into the MHA universe. He teams up with Deku and they face off against new villains created specifically for this crossover. It's a great read for those who like to see different universes collide.