Some of the more popular human characters could be those who form unexpected alliances with the main Helluva Boss characters. A human who teams up with Blitzo, for instance, and helps him with his assassination business in a way that no other demon could. This human might have a unique skill set or perspective that makes them stand out in the fanfiction community.
Perhaps the story could focus on a hellhound and a human forming an unlikely friendship. They could face challenges together, like dealing with the social stigma in Helluva Boss's world of such an odd pairing. The human could introduce the hellhound to human concepts, and the hellhound could show the human the secrets of the hellish realm.
Some of the antagonists from Helluva Boss could also be important characters in the fanfiction. They could try to disrupt the relationship between the hellhound and the human. Maybe a powerful demon who doesn't like the idea of a hellhound getting involved with a human and tries to stop them, creating a lot of conflict and action in the story.
One idea could be exploring the backstories of the minor characters in more detail. For example, delving into Moxxie's childhood and how it shaped him into the imp he is today. Another could be a crossover with another similar animated series, like mixing the Helluva Boss universe with Hazbin Hotel and seeing how the characters interact. Maybe a fanfic could focus on Blitzo's relationship with his family and how it affects his work at I.M.P.
The 'A Night with Loona Helluva Boss' comic is full of surprises and twists. It has a unique story line that takes Loona on a journey through a mysterious and sometimes chaotic night. The illustrations are detailed and really bring the characters to life.
Well, Blitzo is often written about because of his leadership role at I.M.P. and all the comical and sometimes tragic things that happen to him. Stolas is another popular character. His relationship with Blitzo, which is both business - related and has some underlying tension, is ripe for exploration in fanfic. And of course, Loona, Blitzo's adopted daughter, with her moody and somewhat rebellious attitude, is a favorite among many fanfic writers.
Another theme is the contrast between their personalities. Moxxie is more reserved and proper while Loona is brash and moody. Fanfics play with this difference, showing how they either clash because of it or find a way to complement each other. For example, in some fanfics, Moxxie's calmness helps soothe Loona's temper, and Loona's boldness encourages Moxxie to be more adventurous.
One great place to start looking for Helluva Boss Josh x Ace fanfiction is on Archive of Our Own (AO3). There are many stories there with different plotlines. Some might focus on their adventures together in the Helluva Boss universe, while others could explore a more romantic side of their relationship. It really depends on what kind of story you're into.