Well, one reason could be a new start. Maybe she wants to break free from her old self. In some fanfictions, she might cut her hair after a big event like a break - up or a change in her life situation. It could also be a symbol of her growing independence, showing that she's not the same girl she used to be.
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.
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.
Well, it's simply immoral. Incest is not acceptable in society, and fanfiction, while being creative, should also adhere to basic moral principles.
One reason could be for a disguise. Maybe she's on the run from some bad guys in the fanfiction world and changing her hair color helps her blend in better.
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.
She might be portrayed as a villain by showing her being manipulative. For example, she could use her relationship with other characters to get what she wants, like using Edward's love for her to control his actions or decisions.
In fanfiction, Bella might be the Volturi's princess because of a misinterpreted prophecy. The Volturi could believe that Bella is the key to something great, like ruling over all vampires or finding a way to live in the open without fear. So, they give her this princess - like status. Also, it could be due to her relationship with Edward. If the Volturi see Edward as a threat or an asset, they might use Bella to manipulate him, and making her a princess is part of that strategy.
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.
There probably is. Fanfiction writers are very creative and love to cross over different fandoms. Bella Swan's story has elements like love and the supernatural, and the Addams Family has a very distinct and interesting set of characters. Combining them in a fanfiction can lead to some really unique and engaging stories. Maybe it could be about Bella Swan somehow getting involved with the Addams Family's strange and wonderful world.