Perhaps there is a love triangle situation where a character is interested in both Bella and Elena. This causes tension between the sisters at first, but they soon realize that their sisterly relationship is more important than any romantic interest. They then team up to deal with the character who is causing the trouble and in the end, they both find true love elsewhere while still remaining close sisters.
A storyline could involve Bella getting into trouble with some supernatural beings and Elena having to rescue her. Since in many shows or books, there are often supernatural elements. Elena, using her own skills or connections, manages to save Bella and in the process, their sisterly bond becomes even stronger. This could also introduce other interesting characters into the story.
One possible storyline could be that they discover a family secret together. Maybe something about their parents or their family heritage that has been hidden from them. This discovery could lead them on a journey to uncover more about their family's past.
One possible storyline could be that Bella discovers her sisterhood with Jane and tries to use it to protect the Cullens from the Volturi. She might try to negotiate with Jane, using their family bond to sway her decisions regarding the Cullens.
One possible scenario is when Bella is more successful in achieving a common goal in the fanfiction world. For instance, if they are both after a magical artifact, and Bella gets it first. Another scenario could be that Bella has a closer relationship with a mentor figure, which Elena covets. It might also be that Bella has a more glamorous appearance and gets more admiration from other characters, making Elena jealous.
In this fanfiction, they might have a very close and protective relationship. Since they are sisters, Bella could be always looking out for Elena, and vice versa. They might share secrets, support each other through difficult times, and be each other's confidantes.
Perhaps in the fanfiction, Bella could be in a situation where she tries to expose Edward's abusive behavior to the vampire community. This could lead to a complex web of relationships being affected as the vampires have their own set of rules and ethics regarding such behavior. And she may find some unexpected allies within the vampire world who help her get away from Edward.
A possible plotline could be that Bella's new status as Emmett's sister makes her more confident in the vampire world. Since Emmett is a strong and outgoing vampire, he might teach Bella how to better use her vampire abilities if she were to become one in the story. This could lead to her getting involved in more dangerous situations but also growing as a character.
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.
One possible storyline could be that Annabeth uses her strategic skills to help Bella deal with the vampire and werewolf situation in Forks. Maybe she gives Bella advice on how to handle Edward's overprotectiveness or Jacob's pushiness.
One possible storyline could be that Bella discovers her true heritage as Rowena's daughter and starts to learn magic. This could completely change her relationship with the vampires in the Twilight world as she now has a new power to protect herself.
One possible storyline could be that Edward leaves due to his fear of endangering Bella and the unborn child. He might think that his vampire nature could bring harm to them, so he decides to distance himself to keep them safe.