One possible plot could be that Bella and Elijah face challenges from the vampire and werewolf communities. Their relationship might be seen as an oddity or a threat. For example, other vampires could be jealous of Elijah's relationship with Bella, or werewolves might see Bella as a target because of her connection to a powerful vampire like Elijah.
Well, it could involve Bella discovering her true heritage as Elijah's daughter. Maybe there are conflicts with other characters who have different intentions regarding her. For example, some might want to use her for power, while she just wants a normal life.
It could also focus on their personal growth within the marriage. Bella might have always been a bit shy and Elijah helps her come out of her shell. He encourages her to pursue her dreams, like starting her own small business. In turn, Bella helps Elijah become more in touch with his emotions and they both become better versions of themselves through their relationship.
The plot might involve Elijah having some sort of special connection to Bella. He could be her protector by some pre - ordained fate. Perhaps he has been watching over her all along and when the moment of crisis comes, he steps in. He might have to fight off other supernatural beings who are after Bella for some reason, like her unique powers or knowledge she holds.
It could be that Elijah's own family members play the role of antagonists at first. Maybe some of them don't approve of Bella, thinking she is not suitable for Elijah. This could create a lot of internal conflict within the family and also between Bella and Elijah as they try to overcome the family's objections.
Well, one possibility is that Bella's presence changes the dynamic between Klaus and Elijah. They might start to see things from a different perspective because of her. For example, Klaus, who is often seen as the more volatile one, could become more protective of Bella, which in turn makes Elijah more accepting of Klaus's actions. And Bella might introduce them to new ideas or values from her own human experience.
Maybe there are some traditionalists in their world who don't approve of their relationship or the pregnancy. These people could be the antagonists, constantly trying to break them apart or cause problems for them.
One possible plot could be that they are in a forced marriage. Maybe as part of some dark deal between Voldemort and Dumbledore's side. And they have to learn to live with each other, gradually finding unexpected common ground.
One possible plot could be that Harry discovers his true parentage as Elijah's son later in life. This revelation would turn his world upside down and he'd have to re - evaluate his place in the wizarding world.
A possible plot could be that Bella and Elijah are from different species in the fanfiction world, for example, one is a vampire and the other a werewolf (assuming such a scenario exists in this fanfic). Their pregnancy is then a very unique situation that defies the norms of their world. This could lead to a lot of drama, with different factions either trying to protect or harm them because of the baby. They would have to rely on each other and a few trusted friends to navigate this new and dangerous situation.
A potential main character could be Bella's best friend. This friend could provide support and a different perspective as Bella navigates her new identity. There might also be a mentor figure for Bella, someone who knows a lot about Elijah's world and can guide her. This could be an old friend of Elijah's or a wise elder in their community.