A plot could be that Bella discovers a diary or some old records about Katherine. As she delves deeper, she starts to have dreams where Katherine appears and gives her advice. These dreams start to influence her decisions in the real world. She might start to dress differently, act more seductively, and become more assertive. This new Bella could shock the people around her, and she could even use her knowledge of Katherine to outwit some of the more malicious vampires that come to Forks.
Maybe Bella begins to idolize Katherine so much that she tries to emulate her style and behavior. This could lead to a rift between her and Edward, as he doesn't understand her newfound fascination. Bella might then get involved with some of Katherine's old enemies or allies, unknowingly bringing a whole new set of problems to Forks. It could also involve her trying to learn some of Katherine's vampire - related skills, like compulsion, if that was possible in the context of the story.
Her relationship with Edward would definitely change. Edward is used to a more innocent and reserved Bella. If she becomes a Katherine Pierce fan, she might become more assertive and independent, which could either make Edward more attracted to her in a new way or cause him to be wary. He might not like the new, more dangerous side of Bella that is emerging.
One possible storyline could be Bella's struggle to fit in with the Pierce Vampire community. She might face discrimination or challenges because she was originally a human associated with the Cullens. Another storyline could be her search for her true identity as a Pierce Vampire. Maybe she discovers she has a special role or prophecy associated with her new vampiric form. And she could also be involved in a power struggle within the Pierce Vampire hierarchy.
One possible plotline could be that Bella is secretly recruited by a shadowy organization. She has to use her intelligence and her knowledge of the Twilight world's secrets to complete missions. For example, she might use her familiarity with vampires and werewolves to her advantage in spy operations.
One possible plotline could be Bella Swan, now as Bellathena Mikealson, having to navigate the complex vampire - werewolf politics in a different way. Maybe she uses her new identity to bring about a truce between the two groups.
In fanfiction where Bella Swan becomes a Pierce Vampire, her appearance might be different. Pierce Vampires could have distinct physical features. Maybe her eyes turn a different color, or she has a more ethereal glow. Her style of dressing could also change to reflect her new vampiric status. She might be more drawn to darker, more elegant clothing. Her interactions with other characters would be fascinating to explore. How would the Volturi react to her transformation? Would they see her as a threat or an ally? And how would her human friends, like Jessica and Angela, perceive her now that she's a different kind of vampire?
A possible plotline could be that the Winchester sisters are magically transformed into Bella Swan right before a major event in the 'Twilight' series. They then have to quickly adapt to her life and relationships while also trying to figure out a way to get back to their own world. They might use their combat skills to protect themselves from the vampires and werewolves in a more aggressive way than Bella usually does.
Another plotline could be that Bella's transformation into a black witch is part of a larger, darker plot in the Twilight world. Maybe there are forces that are trying to use her powers for their own evil purposes. So she has to not only come to terms with her new identity but also fight against these forces that are trying to manipulate her. This could involve her making some difficult choices between her old life and the new path she's on as a black witch.
Well, a plotline could involve Bella being hunted by some sort of demon hunters who sense her devilish origin. Meanwhile, she tries to understand why her father was involved in hiding her true identity all this time. She could also have an internal struggle between the good in her, which was influenced by her human upbringing, and the evil tendencies that come with being the devil's daughter. There might be a prophecy about her that she has to either fulfill or fight against, and this could be intertwined with the Twilight universe's existing supernatural elements.
Well, a plotline might be that Bella, as the daughter of Ares, has a natural inclination towards fighting and protecting. In the fanfiction, she could use this to protect Forks from some new supernatural threat that is not part of the original Twilight story. Maybe she forms an alliance with some of the werewolves or vampires based on her newfound abilities and the fact that they all need to protect their territory. It could also explore how her relationship with her father Ares, if he is involved in the story, affects her decisions and actions.
One possible plotline could be Bella Swan suddenly discovers she is an Addams daughter. She then has to navigate between her normal life in Forks and her newfound family's rather eccentric and macabre ways. For example, she might struggle with how to introduce her vampire friends to her Addams family who have their own unique view on the supernatural.