Well, in certain fanfictions of New Moon, Bella could become a vampire as a result of a deal. Maybe she makes a pact with a powerful vampire who has the ability to change her in exchange for something valuable, like information or her loyalty. This would be a very different take from the original story, adding an element of intrigue and negotiation to her transformation.
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.
Since Bella is already a vampire, her character development could focus more on her coming to terms with her new existence in a different way. She might not have the same initial struggles with bloodlust as she does in the books. Instead, she could be more interested in exploring the vampire culture and history right away. This would lead her to interact with different vampires, not just the Cullens, and gain a broader understanding of their world. This, in turn, would shape her character into a more worldly and independent vampire.
In fanfiction, the stories can deviate from the original plot. For example, some fanfictions might change the ending. In the original Twilight, the ending was a certain way, but in fanfiction, Bella and Edward could end up in completely different situations.
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.
In Twilight fanfiction, Vampire Bella going to school has multiple difficulties. Firstly, her appearance. Even though she tries to blend in, her pale skin and other vampiric features might make her stand out. Secondly, her academic performance. With her new vampire - enhanced mind, she may find it hard to not seem too smart and draw unwanted attention. Thirdly, the relationships. She has to keep her distance from her old human friends in a way that doesn't seem too odd, while also dealing with any new vampire - related relationships that might form at school.