Obviously, Bella is a main character as the title implies she is the one becoming a vampire. The Cullen family members are also likely to be main characters. Edward, who was in a relationship with Bella when she was human, will surely play an important role. Also, Carlisle, the patriarch of the Cullen family, might be involved in guiding Bella through her new vampire existence.
Perhaps some of the other Cullen family members would be her allies. They are already vampires and could help her adjust to her new powers and the vampire lifestyle. For instance, Alice might use her power of seeing the future to guide Bella in making important decisions. Jasper could teach her how to control her new vampire instincts, like the urge to hunt. And Emmett could be her sparring partner to help her improve her combat skills.
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 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 'bella becomes a badass vampire fanfiction', she could develop unique vampire powers. For example, she might be able to control the minds of other vampires. This would put her in a powerful position within the vampire world. She could also use her new status to change the way vampires interact with humans, making it more peaceful or something like that.
Well, a plotline could be Bella's reaction to no longer being able to eat human food. She might miss the taste of her favorite dishes and have to deal with the cravings for blood instead. Also, there could be a plot where she discovers new vampire powers that are unique to her. This could lead to her being both feared and respected among other vampires.
One possible plotline could be Bella adjusting to her new vampire abilities. For example, she might struggle with her increased strength at first and accidentally break things. Another could be her relationship with Edward changing as they are now both vampires. Maybe they have newfound understanding and experiences to share.
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.
Well, one consequence could be that she has to distance herself from her human friends. Since she's now a vampire, she might be afraid of accidentally harming them. For example, her scent might be too alluring to them, and she could lose control. Also, her family relationships would change. Her father, for instance, would be left in the dark about her true nature, and she'd have to keep lying to him, which would create a strain on their relationship.
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.