One possible supporting character could be Stefan. Since he is Damon's brother in the Vampire Diaries universe, he might play a role in the fanfiction. He could be either supportive of their relationship or a bit hesitant at first.
Well, one interesting plot could be how Bella and Damon first met in the fanfiction world. Maybe it was a chance encounter at a supernatural event. Another plot could involve them facing challenges together, like dealing with other supernatural beings who are against their relationship.
Caroline could be in the story. She might be Bella's friend or a rival. If she's a friend, she could help Bella navigate the supernatural world. If a rival, it could be because of her relationship with Damon or Stefan in some way. Also, Klaus could be a character. He might see Bella as a means to get to Damon, or he could be interested in her unique abilities if she has any.
There could be Bella's mother. She would be an important character as she was part of the relationship that led to Bella's birth. Maybe she has some secrets or reasons for keeping Damon away from Bella all these years.
Damon could be a vampire or some other powerful supernatural being. Bella might be a human, perhaps with some special quality that makes her a target for James. Maybe she has a unique scent or power that she's not even aware of yet.
Well, it could be about Damon having to protect Bella from the various threats in their supernatural world. Maybe she has some unique powers that draw unwanted attention.
In the 'damon is bella's father fanfiction', it could start with Damon having a strange feeling that he has a connection to someone. As he investigates, he finds out about Bella. Their relationship could be filled with misunderstandings at first. For example, Bella might be resentful that he wasn't there for her growing up. But over time, they could learn to understand each other. Damon could introduce Bella to his world, whether it's a world of magic or some other extraordinary place, and they could form a strong bond.
One common theme is family discovery. Since they are twins, there's a lot about uncovering their shared past and family secrets. Another theme is identity. They often have to figure out how being twins changes who they are in the context of the story.
Often, other vampires or supernatural creatures could be the main antagonists. For example, vampires who are jealous of Damon's newfound mate might try to drive a wedge between them. Maybe there are some werewolves who see Bella as a threat to their territory because of her relationship with Damon.