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.
In 'Twilight Vampire Bella fanfiction', a very common theme is the 'what - if' scenarios. For example, what if Bella had chosen Jacob instead of Edward? Or what if she had never become a vampire? Also, the theme of Bella's role in the vampire society is quite popular. How she changes the dynamics, brings new ideas, and faces opposition from the traditional vampire ways. There's also the theme of her family relationships in the vampire world, like how she interacts with the Cullens and other vampire families.
Often, you'll see Edward in these fanfictions. Even though the story is about Bella as a vampire before Twilight, Edward still plays a role. Maybe he's a mystery to Bella at first, or he's from a different vampire group that she has to interact with. Another common character is Jacob. He could be in a more antagonistic role, being jealous of Bella's new vampire life. And of course, the Cullens might be mentioned or even play a part. Carlisle could be a figure that Bella looks up to for advice on being a 'good' vampire.