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 '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.
One interesting plot twist could be that Bella, as a new vampire, discovers she has unique powers that no other vampires have. For example, she might be able to control the elements in some way.
One possible scenario could be that Finn, after being turned into a vampire, struggles with his new identity. He might have to deal with the urge to drink blood while still holding onto his old values. For example, he could try to find alternative ways to satisfy his thirst that don't involve harming humans, like drinking animal blood.
One idea could be that Stiles is turned into a vampire against his will. Maybe he's attacked one night while out investigating something supernatural. As a vampire, he has to struggle with his new urges, like the thirst for blood, while still trying to be the same person his friends know. He might start to see the world in a different light, both literally and metaphorically, as his vampire senses heighten.