There are fanfictions where Bella is a vampire from the start, and she has to teach Edward about the 'true' vampire ways. It flips the original story on its head. She could introduce him to a more savage or ancient form of vampirism that he's never experienced before, leading to a very different relationship dynamic between them.
Some fanfictions focus on Bella as a vampire and her relationship with Jacob. Since she's now a vampire, their dynamic changes completely. Jacob might struggle with his feelings for her, and Bella has to decide if she still wants him in her life in this new form.
One interesting story could be where Jacob tries to convince Bella that she doesn't have to fully embrace the vampire life. He might show her all the beauty of the natural world she's about to lose. For example, he could take her to their old favorite spots in the forest. Another could be a story where Bella, as a vampire, has to protect Jacob from other vampires who see him as a threat because of his connection to her.
One interesting story could be where Bella discovers a new power that is unique even among vampires. It adds a new twist to their relationship as Edward tries to understand this new aspect of her.
One interesting Bella Swan vampire fanfiction could be those that explore an alternate universe where Bella becomes a vampire much earlier. It changes the whole dynamic of her relationship with Edward and the Cullen family. Another could be the ones where she has different powers as a vampire, not just the typical ones. There are also fanfictions that focus on her relationships with other vampires outside of the Cullen clan.
One interesting fanfiction could be where Bella has to protect Edward from other vampires who see him as an easy target because he's human. It adds a new dynamic to their relationship as she uses her vampire abilities in unexpected ways.
One interesting fanfiction could be where Edward and Bella travel to different vampire clans around the world together. They learn about unique vampire cultures and powers, which deepens their bond in a new way.
A great fanfiction idea could be a crossover where Bella Swan has a sister who is a witch like Bonnie in Vampire Diaries. The sister gets involved with the vampires in Mystic Falls, and Bella has to come to terms with this new world while also dealing with her own vampire - human love triangle in Forks.
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.
One interesting plot could be Bella's struggle to control her new vampire instincts. For example, she might have a hard time dealing with her sudden urge for blood while trying to maintain her relationships with her human friends and family. Another plot could be her discovery of new vampire powers that are unique to her. Maybe she can do something no other vampire can, like communicate with animals on a deeper level.
In some 'Bella Swan as a Vampire' fanfiction, a great plotline is her exploration of her new vampire senses. She could find that her sense of smell leads her to discover things about the past of the places she visits. And in terms of relationships, her bond with Edward could change in a really interesting way. Since they are both vampires now, they might have different power dynamics and new challenges in their relationship. For instance, they could compete in terms of their vampire skills, which could either strengthen or strain their relationship.