Billy Butcher has a lot of ups and downs in the comics. He has to deal with personal demons and external threats, which leads to some unexpected developments.
In the comics, Billy Butcher doesn't possess powers. He relies on his combat skills, experience, and a never-back-down attitude to fight against those with supernatural abilities.
One great 'billy butcher fanfic' could be the one where he teams up with an unexpected ally. It shows a different side of his character, exploring his ability to form alliances beyond what we see in the original show. It's full of action and great character development.
First, study the character of Billy Butcher well. Watch the source material closely to understand his mannerisms, his speech patterns, and his values. Then, come up with an interesting plot idea. It could be something related to his revenge arc or his relationships with other characters.
I recommend reading " The Beautiful Female Supporting Character: Marry a Harsh Man in the Age of Book-travel " and " The Counterattack of the Female Supporting Character in Time Travel ". These two novels were both modern romance and urban supernatural novels. They told the story of the female protagonist being locked in the management novel space and starting her journey in the era. Although the plots of these novels are not exactly the same as Miss Butcher's story, they have similar elements and plots that may interest you.