Her relationship with Bella would likely become more complex. As a vampire, she would have different needs and instincts. She might be more protective of Bella in a way that Bella doesn't understand at first. For example, she could sense danger towards Bella that a human wouldn't notice. But at the same time, Bella might be a bit afraid of her sister's new vampire nature, which could create some tension between them.
One plot idea could be that Bella's sister discovers she has a special power as a vampire, like being able to see the future. This could lead to her trying to prevent bad things from happening to her family and the Cullen family. Another idea is that she struggles with the morality of being a vampire while also being in love with a human. She might try to find a way to exist without harming humans, which would create a lot of internal conflict.
One interesting aspect could be how their relationship dynamics change. In the original, Edward was the vampire and Bella was the human, so reversing their roles might lead to a different power dynamic. For example, Bella as a vampire would have more physical strength and speed, which could make her more protective of Edward. Also, Edward as a human might struggle with his mortality compared to Bella's immortality, creating new tensions and storylines in the fanfiction.
One great fanfiction could be where Edward and Bella's relationship is explored more deeply in a different vampire society setting. For example, they might be in a vampire clan that has unique rules about human - vampire relationships.
In such a fanfiction, it could change the entire dynamic of Bella's relationship with the other characters. Maybe she'd be more confident and powerful from the start.
One popular 'Bella is already a vampire' Twilight fanfiction is 'The New Life of Bella Swan'. It takes the concept and explores how Bella's relationships change when she's already a vampire from the start. Another is 'Bella's Vampire Beginnings' which delves into her early days as a vampire, her new powers and how she copes with the vampire lifestyle.
One common theme is the exploration of Bella's transformation into a vampire in more depth. Another is the relationship between Bella and Edward, but often with a new twist. For example, some fanfictions might focus on how they deal with different challenges as vampires together.
A common theme is the 'what - ifs'. What if Bella had chosen Jacob instead? Fanfictions often explore this alternative storyline and how it would affect the vampire - human relations in Forks. It can be really interesting to see how the characters develop in this different scenario.
One unique element could be the different power dynamics. Since Bella is already a vampire, she doesn't have the same vulnerability as in the original story. For example, her relationship with Edward might not be as protective - based. Another element is how she fits into the vampire society right away. In the normal story, she has to be gradually introduced, but here she's already part of it, so it might change the way other vampires view and interact with her.
One popular 'Twilight Bella Vampire Fanfiction' is 'Life and Death' which reimagines the story with gender - swapped roles. It offers a fresh perspective on the Bella - Edward relationship.