I also like 'Kurapika and Kuroro: A Shared Quest'. It's a story where they both embark on a journey to find a powerful and mysterious item in the Hunter x Hunter universe. Along the way, they face many challenges and their interactions are full of tension and some unexpected moments of camaraderie.
One common theme is power. Kuroro is a powerful character, and many fanfictions explore how he gains more power or uses his existing power in different situations.
A good fanfic in this regard often has well - developed characters. Since Kuroro and Kurapika are both complex, their motives, backstories, and personalities should be explored in depth. For example, Kurapika's quest for revenge against the Phantom Troupe and Kuroro's leadership of the troupe are key aspects.
Hisoka fanfic often focuses on his unpredictable nature. Hisoka is a complex character, so many fanfics explore his motives and desires. For example, some fanfics might dig into why he is so obsessed with strong opponents.
Hisoka is still alive in the manga. The storylines haven't led to his death yet. Maybe there'll be developments in the future, but for now, he's kicking.
Another interesting fanfiction is 'Kuroro and the Phantom Troupe Reunion'. It focuses on Kuroro's relationship with the members of the Phantom Troupe. The author does a great job of portraying their complex dynamics and the loyalty they have for each other.