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.
Bella might initially be terrified. She is a human being kidnapped by a vampire, after all. She could struggle and try to escape, using her wits and any available resources. However, since she has feelings for Edward in the original story, part of her might also be conflicted, wondering if there is a good reason behind his actions.
Bella might go into a deep depression. She was so in love with Edward, and his death could shatter her world. She could become withdrawn and isolate herself from her friends and family.
Edward would likely be in a state of panic. He loves Bella deeply, so he'd probably do everything in his power to save her. He might search for a cure or a way to reverse whatever is causing her to die.
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.
Edward would be completely devastated. He would likely go into a blind rage and seek out whoever was responsible for Bella's death. His love for her was so intense that he would stop at nothing to avenge her.