In the fanfic, Fizz could be given new skills or abilities that he has to learn to control. This would create a growth arc for him. For Ozzie, you could show his softer side more often. Maybe he has a hobby or a passion that is not related to his usual tough - guy image in Helluva Boss. This would make him more well - rounded.
One plot idea could be Fizz and Ozzie facing a challenge in their relationship due to external pressures in the Helluva Boss world. Maybe there are other demons who don't approve of their relationship and try to break them apart.
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.
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.
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.
The storylines in Hazbin Hotel might not be directly affected at first. But if the characters start to get influenced by the ideas in the Helluva Boss fanfiction, it could lead to some changes. For example, a character might get inspired by a character's behavior in the fanfiction and start to act differently in Hazbin Hotel.