One reason could be to explore a different side of Bella Swan. Writers might want to break away from the typical good - girl image of her and see how she would function as a more villainous character. It adds a new dimension to the story.
Well, in some fanfictions, she might be depicted as the devil through her actions. Maybe she's shown as being manipulative, using her charm to get what she wants from others, much like how the devil is often portrayed as a tempter in traditional stories.
Perhaps she is deceived. In some fanfictions, she could be tricked into believing that the Volturi are the 'good guys' or that they can offer her something that the Cullens can't. It could be a false promise of safety or a way to protect her loved ones that leads her to become a Volturi member.
One reason could be the allure of the vampire - human relationship in both the Twilight and Vampire Chronicles universes. Writers might want to combine the two to see how Bella's experience with vampires like Edward in Twilight could translate to a relationship with Lestat.
One reason could be for shock value. Writers might want to create something completely different from the normal, loving relationship in the Twilight series to get a strong reaction from readers. Another possibility is that they want to explore the darker sides of the characters, pushing the boundaries of what we know about Bella and Edward.
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.
Power could be a motive. She might want to have control over the vampire world or the humans in Forks. So she starts doing villainous things to gain that power.
One possible reason is that the writer wants to explore the psychological aspects of such an unequal relationship. How would Bella feel, being in a position of slavery? How would Edward handle this extreme power? But it's important to note that this concept is quite controversial and not in line with the general positive tone of the 'Twilight' story.
One possible storyline could be that Bella discovers her Winchester heritage and has to learn how to hunt supernatural creatures like the rest of the family. She might struggle at first, but with the help of Sam and Dean, she becomes a skilled hunter.
A really interesting storyline could be Bella trying to reconcile her old life in Forks with her new life as a Mikaelson. She might struggle with the differences in values and lifestyles. For example, the Mikaelsons are more into power plays and manipulation in the supernatural world, while Bella was used to the more straightforward (although still complex) relationships in 'Twilight'. This internal conflict could be a major part of the story, along with her trying to find her place within the Mikaelson family and the wider supernatural community.
Well, it's simply immoral. Incest is not acceptable in society, and fanfiction, while being creative, should also adhere to basic moral principles.