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.
A more character - centric plot could be about Rosalie and Bella finally coming to a true understanding and friendship. Alice could be the catalyst in this, arranging situations where they have to work together and learn about each other's pasts. This could include Rosalie sharing her human life story with Bella, and Bella sharing her fears and hopes for her new vampire life.
One plot idea could be that Rosalie and Bella discover a hidden power within their futa forms that is connected to the supernatural world around them. They then have to navigate how to use this power while also dealing with the reactions of their friends and family in the Twilight universe.
One plot idea could be a story where Bella, Rosalie, and Alice have to work together to solve a mystery in the vampire world. Maybe there's a new threat that only their combined skills can handle. Another could be a story focused on their friendship, like a road trip where they discover new things about each other and themselves. Or perhaps a story where they use their powers to help humans in a small town without revealing their true nature.
In twilight Rosalie and Bella fanfiction, a great plotline could be Rosalie helping Bella adjust to her new vampire life in a more motherly way. Since Rosalie has been a vampire for a long time, she has a lot of knowledge and experience. Maybe she could help Bella with controlling her new powers and dealing with the emotional changes. There could also be a plot where they go on a journey together, away from the Cullen family for a while, to discover more about themselves and their place in the vampire world. This journey could bring them closer and make them see each other in a whole new light.
Well, a common plotline is that they discover their mate bond in a rather unexpected way. For instance, in some fanfictions, there could be a magical event or a prophecy that reveals they are mates. Once they realize this, they have to deal with the complex emotions that come with it. Rosalie, who has always been protective of her family and had a certain image, might struggle with accepting her feelings for Bella. And Bella, who has been through so much with the Cullens already, now has to navigate this new relationship dynamic. It could also lead to some interesting power dynamics within the Cullen family as they adjust to Bella and Rosalie being mates.
One idea could be that Bella discovers Rosalie's possessive nature towards her new vampire life. Rosalie might try to control Bella's actions to prevent her from making what she sees as mistakes. For example, Rosalie could be possessive over the 'right' way to be a vampire and try to keep Bella from straying from it.
A third plot might involve a mystery in the modeling world. Bella, Alice and Rosalie are models, and suddenly, there are strange disappearances of models from their agency. They decide to investigate. As they dig deeper, they find that there is a jealous rival trying to sabotage their agency. They use their different skills - Bella's tenacity, Alice's intuition and Rosalie's beauty and charm - to expose the culprit and save their agency.
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.
Maybe in a fanfiction, Demetri discovers that Bella has a hidden power that not even the Cullens are aware of. He then has to decide whether to report it to the Volturi or keep it a secret and help her develop this power. This could create a complex relationship between them, full of trust issues and moral dilemmas.
One plot idea could be that Felix, instead of being a villain, becomes Bella's secret protector in the Twilight world. He might sense some unique power within her that even the Cullens are unaware of. This could lead to a series of adventures where they have to hide their alliance from both the Volturi and the Cullens.