One great Gon x Killua x Hisoka fanfiction is 'Unlikely Alliances'. It explores their complex relationships in a unique adventure setting. The story does a wonderful job of showing how Gon's pure heart, Killua's loyalty, and Hisoka's unpredictability interact in different situations.
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.
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 common theme is the exploration of power dynamics. Hisoka is much more powerful and experienced, and Killua is a prodigy. Their relationship often involves Hisoka testing Killua's limits. Another theme is the idea of redemption or change. Hisoka is a rather amoral character, and in some fanfics, Killua might influence him to change for the better.
There are quite a few out there. Some might focus on a moment of deep emotional connection between them that leads to the kiss. You can start by looking on fanfiction platforms like Archive of Our Own. Just search for 'Killua and Gon kiss' and browse through the results to find ones that have good ratings and reviews.
A very common theme is the exploration of unspoken emotions. In the original 'Hunter x Hunter' series, there are a lot of feelings between Killua and Gon that are not fully expressed. In fanfiction with a kiss, it's a way to finally let those emotions out. There can also be the theme of forbidden love in some cases, where they might face challenges or opposition to their relationship, and the kiss is a rebellious act against those constraints.
One theme that shows up a lot is sacrifice. Killua's willingness to sacrifice for Gon and vice versa is a big part of their relationship. In fanfictions, this can be magnified, with one sacrificing in a more extreme way for the other's well - being or for the sake of their relationship. Also, self - discovery is a theme. As they grow closer, they might discover new things about themselves and their feelings for each other.
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.