The themes could include the journey of them coming to terms with their feelings for each other. Also, the idea of Ging's past and how it affects their relationship with Gon could be a major theme. There might also be themes about how their relationship impacts the world around them in the Hunter x Hunter universe.
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.
I'm not sure specifically which Ging meets Gon in a 'boyfriend' context fanfictions are out there. You could try searching on popular fanfiction websites like Archive of Our Own or FanFiction.net. Just type in relevant keywords and browse through the results.
Hisoka's origin story involves his early days as a powerful and somewhat unhinged nen user. Key events include his discovery of his nen abilities and his growing fascination with strong opponents. He has always been on a quest to find and battle those who can give him a good fight, which led him to various parts of the world in search of worthy adversaries.