Well, for one, the whole concept of their courtship would be different. Since Bella is now the vampire, she would be the one with the enhanced senses and abilities. She might be more forward in the relationship as she doesn't have the same insecurities as she did as a human. Edward, on the other hand, might be more hesitant and in awe of Bella's newfound power.
The danger aspect would also shift. In the original story, Edward was constantly worried about Bella's safety from other vampires. But now, it could be Bella who has to protect Edward from the vampire world. She would have to use her vampire skills to keep him safe. And Edward would have to deal with the knowledge that there are these powerful, immortal beings out there that his vampire - girlfriend is a part of.
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 popular fanfiction could be about Bella using her vampire abilities to protect Edward from some new supernatural threat in Forks. Maybe she has to face a powerful vampire coven that has a vendetta against the Cullens, and in the process, her relationship with Edward deepens in new and unexpected ways.
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.
The Cullen family might also get involved in different ways. Some of them could be sympathetic towards Bella and offer their help, while others might be more hesitant due to the implications of a half - vampire baby. Esme, for example, might be very motherly and try to take care of Bella, while Rosalie could have mixed feelings because of her own history with not being able to have children.
A lot could happen. Edward could have some clashes with the vampires in the Forbidden Forest, if there are any. Bella might make friends with some of the more curious students at Hogwarts, like Hermione. And perhaps there would be a situation where Edward's mind - reading ability is either an advantage or a hindrance in the magical world. For example, he could read the minds of those planning to expose him, or it could be blocked by some powerful magic spells.
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.
Another good one is 'The Vampire's Mate'. It delves deeper into the vampire lore and how Bella, as a human, fits into their world. The relationship between Edward and Bella is portrayed in a more intense and detailed way, which makes for a very engaging read.
One common theme is the exploration of power dynamics. Since Bella is a vampire and Edward is human, it changes the traditional power balance in their relationship. For example, Bella may have to protect Edward from other vampires.
Love against all odds is a very common theme. Since they are from different species, there are a lot of obstacles in their relationship. For example, in many fanfictions, Bella has to control her vampire instincts around Edward, which adds a layer of tension.
Another common theme is the struggle between the vampire and human worlds. There are often conflicts arising from the differences in their natures, values, and lifestyles. Fanfiction writers usually show how Edward and Bella deal with these differences and try to find a balance between the two worlds.