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.
I'm not sure if there is such a specific fanfic. There are countless fanfics out there, and while the 'Twilight' series has a large fanfic community, this particular combination seems rather unorthodox. It might exist in the vast realm of fan - created works, but it's not a common or widely - known trope within the official 'Twilight' lore.
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.
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.
One common theme is the exploration of their love in different situations. For example, in some fanfictions, they face new threats to their relationship, like a new vampire clan that doesn't approve of their union. Another theme is the power dynamics between them. Since Edward is a vampire with special abilities and Bella is a human (at least initially), how they navigate that power difference is often explored.
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.
Love is a very common theme. Their intense, often obsessive love for each other is a central part of most fanfictions. Another theme is the struggle between Bella's humanity and the vampire world she's drawn into. Also, the idea of family, like Edward's vampire family and how Bella fits into it.
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.
You could make their relationship more unique by having them communicate in a different way. For example, instead of just talking, they can have a mental connection from the start that only they can understand. This would make their bond more special and exclusive.
There are quite a few great ones. Some focus on the unique challenges of their relationship, like how Bella copes with Edward's vampiric nature while being human. For example, 'A Different Kind of Love' shows their relationship evolving in a very heart - warming way.