The Cullens are very likely to be involved. Edward, of course, would have a huge role as Bella's former love interest. Carlisle, with his knowledge and experience as a vampire, could help Bella adjust to her new life. Alice might use her precognitive abilities to foresee any problems Bella could face. Also, Jacob could be in the story. His reaction to Bella becoming a vampire would be interesting, considering his past relationship with her and his werewolf nature.
One interesting plot development could be Bella struggling to control her new vampire powers. For example, she might accidentally use her super speed and crash into things at first. Another could be her dealing with the changed relationships. Since she's now a vampire, her connection with her human friends and family becomes more complicated as she has to hide her true nature from them. Also, there could be a plot where she discovers new vampire abilities that are unique to her, which leads to her having a special role within the vampire community.
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.