One plot idea could be that Bella's sister discovers Caius's true nature as a vampire but instead of being afraid, she is intrigued. She starts to secretly study vampires and their lore, and in the process, catches Caius's attention. He then decides to keep an eye on her, leading to a series of tense yet fascinating encounters.
Maybe Bella's sister has a special ability that Caius is interested in. For example, she can see glimpses of the future related to vampires. Caius, being one of the Volturi leaders, wants to use this ability for his own purposes. This could lead to a power struggle between them as she tries to resist his control while also being drawn to his mysterious and powerful presence.
There could be a story where Caius has a change of heart after meeting Bella. In the Twilight world, Caius is known for his strict and often cruel ways. But when he encounters Bella, he starts to question his own beliefs. Maybe Bella gets lost in the Volturi territory and Caius finds her. Instead of punishing her, he is intrigued by her. This could lead to a story of him trying to understand her human - vampire relationship with Edward from a different perspective. He might even start to envy Edward in a way, which could create a whole new dynamic in the fanfiction.
Well, perhaps Bella's sister is a half - vampire herself. She was born from a vampire who didn't fully turn her mother. This sister could struggle with her identity, not fully fitting in with either humans or vampires. She might be drawn to the Cullens because of their 'vegetarian' lifestyle but also be scared of them. She could also be friends with Jacob before Bella even arrives in Forks, and Jacob has to keep her secret from Bella until the right time.
One plot idea could be that Bella's sister discovers she has a special power as a vampire, like being able to see the future. This could lead to her trying to prevent bad things from happening to her family and the Cullen family. Another idea is that she struggles with the morality of being a vampire while also being in love with a human. She might try to find a way to exist without harming humans, which would create a lot of internal conflict.
One idea could be that the soulmate connection defies the natural enmity between the Twilight world and the Mikaelson's world. Maybe the soulmate bond allows them to communicate telepathically across different realms. For example, Caius could be in his vampire world in the Twilight universe while his soulmate is in the Mikaelson's world of witches and vampires, and they share dreams and thoughts that gradually bring them closer.
One idea could be that Bella's sister discovers a hidden power within herself. Maybe she can communicate with animals and uses this ability to protect the town from an unknown threat.
It might be that Caius is on a mission outside of Volterra. He crosses paths with Bella's sister in a small town. She is doing some mundane task like shopping, and he is there to hunt (not for her, of course). But her scent catches his attention, and he approaches her. She is shocked to see such a pale and intimidating figure, and this is their first encounter.
Another plot could be that Sam's sister falls in love with a character from a different faction in the Twilight universe. This would create a lot of tension as their relationship would be frowned upon by both of their respective groups. It could explore themes of forbidden love and loyalty, as she has to decide between her family and her newfound love.
Well, it could start with Bella being captured by Caius. As his pet, she is forced to live in his lair. Caius might be possessive of her and not allow her to interact with others. Bella, being a strong - willed character, tries to resist this new role. She could start to learn about Caius' past and weaknesses in order to find a way to break free. There could also be a love - hate relationship developing between them, where Caius starts to have unexpected feelings for Bella as more than just a pet, which further complicates the situation.
One idea could be that Bella's cousin comes to Forks for a visit. Caius, while on some Volturi business in the area, catches sight of her. He's intrigued by her different scent compared to the other humans in town. Their relationship could start with some tense encounters as Caius tries to figure out why she's so interesting to him.
One idea could be that Bella and Rosalie have to team up to protect the Cullen family from a new vampire threat. They start off disliking each other but as they face challenges together, they develop a mutual respect.