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.
Edward might also become desperate. He could reach out to other vampires or supernatural beings for help. He may even consider turning Bella into a vampire against her will if it means saving her life, although this would be a very difficult decision for him as he knows Bella has been hesitant about becoming a vampire in the past.
He could be filled with guilt. If he feels responsible for her condition, he'll be tormented by his thoughts. Maybe he'll isolate himself from others as he tries to figure out a solution, all the while being consumed by his love and worry for Bella. He may also become more reckless in his attempts to save her, not caring about the consequences for himself.
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 She could become withdrawn and isolate herself from her friends and family.
There are other fans who are interested in exploring different storylines. They see it as a creative way to expand the Twilight universe. They might enjoy seeing how the characters would react in such a difficult situation, even if it's not in line with the original story.
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.
She might be devastated. Bella was deeply in love with Edward, so his leaving would be a huge blow. She could cry for days and feel like her world has ended.
Together, they would likely try to make Renesmee understand that their relationship is special and that her presence is also special. They might give her a hug or some form of affectionate gesture to show that they are a family unit and that her walking in is just a normal part of family life, even if it was an intrusion on their private time.
One possible reason could be a vampire attack gone wrong. Maybe a rogue vampire targeted Bella and inflicted a near - fatal wound. Another reason might be related to some sort of dark magic or a curse that was unknowingly placed on her. It could also be due to a self - sacrifice situation where she put herself in harm's way to save someone she loves, like Edward or one of her friends.
There's also 'The Redemption of Edward Cullen'. This fanfiction explores Edward's past in more detail and how it affects his relationship with Bella. It adds new layers to his character and gives a fresh perspective on their love story.
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.
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.
In many Twilight fanfictions, Edward and Bella's kiss is often depicted as an extremely passionate and intense moment. Some fanfictions might explore different scenarios around this kiss. For example, it could be set in an alternate universe where they meet under different circumstances before the kiss. The kiss in fanfiction can also be used to show a deeper connection between them than what was shown in the original books. It allows the writers to play with the emotions and the relationship dynamics in a more creative way.