Well, perhaps in the fanfiction, Bella's transformation into a banshee is related to her love for Edward. Her intense emotions somehow trigger the banshee side of her. As a banshee, she could use her wail to warn the vampires of approaching threats from other supernatural beings. The story could then revolve around how she uses her new abilities to protect the ones she loves and also how the vampire community reacts to this new form of her.
One possible plot could be that Bella discovers her new banshee powers after some strange and traumatic event. Maybe she starts having premonitions not just about danger to herself like in the original Twilight series, but about disasters that will befall the entire town. This could put her in a position where she has to figure out how to control these powers and also how to convince others, like the Cullens, that she's not going crazy.
If Bella becomes a banshee, her relationship with Edward would be very different. He would be more protective of her as she has a new and potentially dangerous power. He might also be a bit wary as banshees are known for their wailing which could attract unwanted attention. With Jacob, he might see her as even more of an enigma and be more drawn to her in a way, since she's now a more powerful supernatural being.
One possible plot could be that Harry, as a banshee, uses his newfound abilities to warn the wizarding world of impending dark magic threats. His banshee screams might be able to disrupt dark spells or alert people to the presence of Death Eaters. For example, he could sense Voldemort's movements in a more acute way and use his screams to drive away the dark forces when they are about to attack Hogwarts.
She might start imagining herself as an assassin in the Twilight world. Maybe she'd use her vampire speed and strength in new, assassin - like ways to protect Forks or her loved ones.
Maybe Bella Swan discovers Cortez's work and it changes her perspective on something. For example, if Cortez is an artist, his art could inspire Bella in her own life in Forks.
Well, it could be that Bella is secretly an assassin all along in Forks. She's using her relationship with Edward as a cover while she's on a mission to take out some supernatural threats in the area.
One possible plot could be that Bella Swan, in this fanfiction, becomes a huge admirer of the Gilbert family. Maybe she is intrigued by their history in the town. She could start to study their family tree, trying to understand the secrets they hold. This could lead her to befriend members of the Gilbert family and get involved in their daily lives, which might be full of both normal family drama and some supernatural elements as in the original 'Twilight' and 'The Vampire Diaries' universes where the Gilberts are from.
Maybe Bella Swan could be transported to Asgard. She might be mistaken for a powerful sorceress because of her unique connection to the Twilight world. Then she has to navigate the politics of Asgard while trying to find a way back home.
One possible plot could be that Bella is somehow transported to hell against her will. Once there, she discovers she has some latent power that makes her a threat to the existing power structure. So, the other demons or rulers in hell try to either eliminate her or make her submit. But Bella, being strong - willed, decides to fight back and rise to the position of princess by overthrowing the cruel rulers.
The plot could involve Bella being raised in the Twilight world without knowing her real mother. Then, through some magical event, she finds out she's Bellatrix's daughter. This could lead to a conflict as she tries to figure out where she truly belongs, between the two very different worlds of vampires and wizards. Also, there could be a plotline where she has to face the dark legacy of her mother and decide whether to embrace or reject it.
Well, it could involve Bella having special powers from being Poseidon's daughter. Maybe she can control water to protect Forks from some new supernatural threats. She might also have a connection to the ocean that draws her away from the normal vampire - human drama in the original 'Twilight' story.