In such a fanfiction, Bella might experience a whole new life. She could gain new powers and abilities as a vampire. For example, she may have enhanced strength and speed. She might also face new challenges in the vampire world, like dealing with other vampire clans or trying to control her new thirst for blood.
In some fanfictions, Edward leaves Bella because he's cursed. He doesn't want Bella to be affected by the curse. Bella, being in love with him, tries to find a way to break the curse. She travels far and wide, meets different supernatural beings, and in the process, she also becomes more powerful. When she finally finds a way to break the curse, Edward realizes how much she truly loves him.
Bella's character might become more confident. Without Edward, she has to rely on herself, which forces her to grow. As a vampire, she'll be more physically capable, and this could translate into mental strength as well.
There's a fanfiction called 'Left Behind'. Here, Edward leaves Bella thinking he's losing his humanity and becoming a danger to her. Bella refuses to accept this and tries to bring him back to his senses. She delves deep into the history of vampires and their emotions to find a solution. It has a very emotional and engaging plot.
Edward would probably be very confused at first. He would try to understand why Bella would leave him for Rosalie. He might start to analyze all their past interactions, looking for signs that he missed. Eventually, he could become angry, not at Bella directly, but at the situation. He might also feel a sense of betrayal, not just from Bella but also from Rosalie in a way, since Rosalie is 'taking' Bella away from him.
It could be that there are external forces at play. Maybe there are other characters or some kind of dark secret in the fanfiction lore that forces Edward to leave Bella while she's pregnant. For example, there could be a threat from a rival group that says they'll harm Bella and the baby if Edward stays, and he thinks leaving will draw the danger away from them.
Edward might be initially shocked. He has always seen Bella as his own and this sudden change could be hard for him to accept. He could become withdrawn and moody, similar to how he was before Bella entered his life.
Edward might be initially shocked and heartbroken. He has been in love with Bella for a long time, so her leaving for Jasper would be a huge blow. He could become withdrawn and moody, brooding over what went wrong.
She could be heartbroken. After all, she's pregnant and the person she loves has left her. She might feel abandoned and alone, crying a lot and wondering why Edward would do such a thing.
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.