Maybe Carlisle and Bella could bond over their shared experiences of being different from the norm. Carlisle is a 'vegetarian' vampire, and Bella is a human who is in love with a vampire. They could have deep conversations about how they both struggle to fit in with their respective worlds and find solace in each other's understanding. This would add a new layer to their relationship and make it more unique.
In a fanfiction, their relationship could take an unexpected turn if Carlisle was somehow put in a position where he had to rely on Bella for his own safety. This reversal of the typical protector - protected dynamic would force them to see each other in a new light. Bella could use her knowledge of the human world and her wits to keep Carlisle out of danger, and in the process, their relationship could grow in a very different and interesting way.
They could develop a mentor - mentee relationship. Carlisle, with his vast knowledge and experience, could teach Bella about the history and culture of vampires. Bella, in turn, could offer Carlisle new perspectives from her modern - day human upbringing. This kind of exchange would strengthen their relationship in a non - romantic way. For instance, they could have long discussions about how vampires have adapted over the centuries and how modern society affects their existence.
One way to develop their relationship uniquely is to put them in a situation where they have to protect each other's secrets. For example, Jasper might have a secret from his past as a vampire in the military, and Bella has her own secrets about her relationship with the wolves. By relying on each other to keep these secrets, a strong bond forms between them.
Their relationship could develop uniquely by having them communicate through a special bond that is exclusive to their futa state. This bond allows them to share thoughts and feelings more deeply than normal, which in turn brings them closer together.
Their relationship could be developed through a shared mission. For example, they are both tasked with protecting a special, ancient artifact that has the power to bring peace to their world. Working towards this common goal, they gradually learn to respect each other's abilities. Alec might be impressed by Seth's loyalty to his pack, and Seth could come to admire Alec's strategic thinking as a vampire.
Their relationship could be developed uniquely by having them communicate through a special form of magic that only they share. This magic could be tied to their emotions, so when they're happy, it creates beautiful glows, and when they're sad, it sends out a signal that only the other can understand.
One way their relationship could develop uniquely is through a shared interest in art. Suppose Bella has an appreciation for ancient vampire art that Caius is also fond of. They start to bond over discussing different artworks and the stories behind them. This common interest bridges the gap between them, and Caius begins to see Bella not just as a human or Edward's mate, but as an individual with her own charm. As their relationship grows, Caius might even start to protect Bella from the Volturi's harsher rules because of this newfound connection.
Their relationship can be developed through shared experiences. For example, they could face a common enemy together. This would force them to rely on each other and understand each other's strengths and weaknesses.
Their relationship could be developed uniquely by having them communicate in a special way. For instance, instead of just talking, they have a mental connection that only they share. This connection allows them to understand each other's feelings and thoughts deeply, which helps them face the challenges in the gp.
One more way is if Bella is an artist. She starts painting portraits of the Lost Boys without knowing their true nature at first. When David sees her paintings, he is drawn to her creativity and the way she seems to capture their essence. This leads to a series of interactions where they slowly open up to each other, with Bella gradually accepting David's vampire world while David discovers the beauty in Bella's art and her human side.
To develop Roseline uniquely, you could give her an unusual hobby like collecting antique keys. This hobby could tie into the story in some way, like leading to a mystery she and Bella have to solve. For Bella, you could make her really good at parkour, which gives her a different kind of physicality and confidence. When they face challenges, their skills can play important roles.