One idea could be that Bella is a sweet - natured girl who is caught in the middle of Damon's wild ways and Stefan's more reserved nature. She might look up to Stefan as a role model for being good, but at the same time be intrigued by Damon's sense of adventure. In the fanfiction, she could end up having to choose sides when a big crisis hits their family, like an attack from a powerful vampire enemy.
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.
Well, one possibility is that Bella's presence completely changes the dynamic between Stefan and Damon. Maybe she's the mediator in their constant feuds. Since she's their sister, she might have a special power that could either help or hinder them in their vampire world. For example, she could have a unique ability to sense danger that the brothers don't have, which would make her an important asset in their battles against other supernatural beings.
One common theme is the struggle for acceptance. Since Bella is the mate of both Damon and Stefan, there's often a plotline about how the vampire community or their friends accept this unusual relationship. For example, they might face judgment from other vampires who don't understand this triad.
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.
If Bella was Stefan and Damon's sister, her relationship with Elena would be very interesting. Elena might see her as a sister - in - law of sorts, but also as a potential rival for the Salvatore brothers' attention. With Caroline, they could become good friends as Bella might be more understanding of Caroline's transformation into a vampire. And with Klaus, he might see her as a new target or an interesting addition to his games, since she's related to the Salvatores.
One possibility is that the baby has some special powers. In the fanfiction, this could lead to a whole new set of adventures. Bella and Stefan could be on a journey to understand and control these powers while also dealing with their own relationship and the new family dynamic.
In this fanfiction, there might be some werewolves who view the baby as an abomination since it's part vampire. They could try to harm the baby, which would put Bella and Stefan in a difficult position to defend their child.
One great fanfiction could be 'Bella's Bond with the Brothers'. It explores Bella's relationship with Damon and Stefan in a really interesting way, showing how she fits into their world of vampires and the unique dynamics that develop among them.
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.