Power imbalance is a common theme. One character may have more control or influence over the other, leading to a tumultuous relationship. For example, in some novels, one character might be wealthy and use that to manipulate the other.
Gray Fullbuster can often be portrayed as a bad boy in fanfiction. His cool and sometimes aloof attitude, along with his stripping habit which can be seen as a bit of a rebel trait, makes him a popular choice. Another one could be Gajeel Redfox. He was originally an enemy but joined Fairy Tail. His rough past and tough exterior give him that bad boy vibe.
He is likely to be a complex character. He may seem tough on the outside with a bit of a rebellious streak. Maybe he has a leather jacket and rides a motorcycle kind of look.
There is 'Painter of the Night'. It has elements of a 'bad romance' in the sense that the relationship between the characters is filled with misunderstandings, power imbalances, and a lot of emotional upheaval. The art in the novel is quite beautiful though, which is an interesting contrast to the rather complex relationship dynamics.
It's not uncommon for such a twist in BL manga. The boy next door could totally be a vampire if the story is set up that way to add excitement and mystery.
The bad boy is a hacker. He uses his skills to break into computer systems just for the thrill of it, not for any malicious intent really. But then he gets targeted by a real cyber - criminal who wants to frame him for a major cyber - crime. He has to use his hacking skills to clear his name and in the process, he teams up with a group of ethical hackers and starts to use his abilities for good.
One idea could be a bad boy who is constantly getting into trouble at school. He skips classes, plays pranks on teachers, but deep down he has a good heart and is just acting out because of family issues at home, like his parents being too strict or not giving him enough attention.