Often, PFPs featuring Sasuke with a determined expression or in a dynamic pose are in demand. Maybe it's because they convey his strength and personality. Also, those with high-quality art and vivid colors get a lot of attention.
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.
Sure. There was an artist who specialized in creating PFPs with a surreal and dreamy style. One of his PFPs got shared by a major digital art collector on a popular art platform. This led to a flood of requests for his work. He was able to sell many of his PFPs as limited - edition digital art pieces and made a name for himself in the digital art community.