Often, there are new vampire characters. Maybe a vampire who was once a famous historical figure, like a medieval knight turned vampire. There could also be a witch - vampire hybrid who becomes Bella's friend or foe. Another common addition is a vampire with a special power related to the mind, who tries to influence Bella after her transformation.
In many fanfictions, new characters include a vampire elder who has different ideas about how new vampires should be trained. This elder might see Bella as a threat or an opportunity. Another common character is a vampire with a connection to an ancient prophecy. When Bella turns, this character believes that she is a key part of the prophecy. Also, there could be a vampire who was exiled from their coven and is looking for a new place in the vampire world, and Bella's transformation draws their attention.
Often, there are midwives or healers that are introduced. These could be ancient vampires with special knowledge of hybrid births. Another common new character is a seer or prophet who can predict the future of Bella's baby. Sometimes, there are also new werewolves who come to protect Bella during the birth because they sense a great change is coming.
Often, you'll see Edward in these fanfictions. Even though the story is about Bella as a vampire before Twilight, Edward still plays a role. Maybe he's a mystery to Bella at first, or he's from a different vampire group that she has to interact with. Another common character is Jacob. He could be in a more antagonistic role, being jealous of Bella's new vampire life. And of course, the Cullens might be mentioned or even play a part. Carlisle could be a figure that Bella looks up to for advice on being a 'good' vampire.
Jacob could also be a common character. Even though Bella has become a vampire, her past connection with Jacob and the werewolf pack might still play a role. There could be some tension or new interactions between the vampires and the werewolves. And of course, Carlisle, as the head of the Cullen family, is likely to be in many of these stories to guide Bella in her new vampire life.
In certain fanfictions, a new hybrid character that is part - vampire, part - something else (maybe an ancient mythical creature) is introduced. This character challenges the existing norms of the Twilight universe. Also, a new human character who is immune to vampire compulsion in a different way than Bella was can be really intriguing. They might use this immunity to their advantage in various ways. And then there are characters who are descendants of long - forgotten vampire or werewolf lines, and their presence stirs up old rivalries and new alliances.
Obviously, Edward is a key character. He is not only Bella's love interest but also someone who has a lot of knowledge about the vampire life. He can help Bella understand her new powers and limitations. Alice is also popular in these fanfictions. With her ability to see the future, she can foresee any potential problems or challenges that Bella might face as a vampire. And then there's Rosalie. Her initial dislike for Bella might change or intensify after Bella turns, adding more drama to the story.
One common theme is the exploration of Bella's transformation into a vampire in more depth. Another is the relationship between Bella and Edward, but often with a new twist. For example, some fanfictions might focus on how they deal with different challenges as vampires together.
There could be other young witches or wizards who are also on the run from something or have their own issues with the wizarding establishment. They bond with Harry over their shared experiences and together they face new challenges. Maybe a young witch who has unique powers related to nature and she helps Harry understand the more elemental aspects of magic.
One interesting plot twist could be that Bella's transformation into a vampire gives her a unique power that no other vampire has. For example, she might be able to control the weather. Another twist could be that instead of joining the Cullen family right away, she runs off on her own to explore the vampire world. Maybe she discovers a whole new coven that has different values and ways of life compared to the Cullens.
Edward would still be a very important character. Since he's Bella's love interest in the original 'Twilight' series, in the fanfiction, he would play a major role in Bella's new vampire life. He might help her adjust to her new powers and the vampire lifestyle.
To adapt to her new vampire life in the fanfiction, Bella has a long journey. At the beginning, she struggles with her speed. She might accidentally run too fast and crash into things. But with Alice's guidance, she learns to control it. Her vision also changes, and she has to learn how to focus in different lighting conditions. Socially, she has to interact with other vampires. Some might be friendly, like those in Alice's coven, but others could be hostile. She has to learn how to defend herself and navigate the complex vampire politics. She also has to come to terms with the fact that she'll live forever now, which means leaving behind her human concept of time and mortality.