In the fanfiction, perhaps the baby's birth is a very difficult and dangerous process. Since both Bella and Damon are from different fictional universes with their own set of rules regarding vampires and such. The baby could be a symbol of a new beginning for their relationship, forcing them to put aside their differences and work together to protect their child. They may have to face challenges from both the vampire and human communities who are either afraid of or envious of the baby.
The baby might have Bella's big, expressive eyes and Damon's dark, tousled hair. Maybe it would inherit Bella's pale complexion and Damon's sharp facial features. Since they both have a certain allure in their respective shows, the baby could be extremely beautiful or handsome, with an otherworldly charm.
Bella might be very empathetic. Since she's in a family full of complex emotions, she can sense when Damon is feeling particularly broody or when Stefan is stressed. So she tries to comfort them in her own way, like making them a special meal or just sitting and listening to their problems.
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.
One idea could be that Damon and Bella meet in a casino in Vegas. Damon, with his charm, tries to win Bella over in a game of poker. Another could be they both get involved in a mystery related to an old Vegas hotel, and they have to work together to solve it.
The baby could also be a way to explore the more human side of Damon. He has always been a bit of a bad boy, but with a baby, he might become more nurturing. Caroline, on the other hand, could be the strict but loving mother. Their different parenting styles could lead to some humorous and heartwarming moments in the fanfiction.
Another plot could be centered around the baby's upbringing in a world full of supernatural beings. Damon and Elena might struggle with how to raise the baby in a way that it can fit into both the human and supernatural worlds. They could face challenges like finding the right school or dealing with other vampires who might not be so accepting of a half - vampire baby. Maybe they would have to move to a new place to keep the baby safe.
It changes drastically. Usually, the trust is broken. Bella may become very guarded and distant towards Damon.
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.
Well, clearly Damon is one of the main characters. He might be the one bringing in elements of mystery, power (if he has any special abilities in the context of the story). The OC, on the other hand, is the female lead who is going to be the mother of their baby. She could be someone with a unique personality, maybe she's strong - willed or very kind, and together they form the core of the story.