A unique development in their relationship could be based on a situation where Bella saves Caius from a dangerous situation. For example, a group of rogue vampires attacks the Volturi, and Bella, using her knowledge of the forest around them (from her time in Forks), helps Caius escape. This act of heroism makes Caius see Bella in a new light. He starts to respect her and wants to learn more about her. He then begins to court her in his own, rather old - fashioned vampire way. He might bring her rare gifts, like ancient vampire relics, and share stories of his long life with her, gradually building a deep and unusual relationship.
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.
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.
A unique way for their relationship to develop is through a shared interest in art or music that neither of them expected. Maybe Bella discovers that Jasper has a hidden talent for painting or playing an instrument, and this new side of him draws her in even more. They start to create art together or attend music events, and through these shared experiences, their love as soulmates blossoms in a way that is distinct from the typical vampire - human relationships in the story.
At first, it might be a bit rocky. Bella, being human and now blind, is in a very vulnerable state. Caius, as a powerful vampire, may come off as intimidating. But as time goes on, they start to develop a deep bond. Caius learns to be more gentle and caring, and Bella starts to trust him more and more.
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.
In a Bella and Caius married fanfiction, their relationship might develop in a rather complex way. Caius is known for his harsh and traditional views in the Twilight universe. But with Bella, he could soften. Maybe at first, their marriage was arranged for some political or vampire - related reasons in the vampire world. Bella, being human - turned - vampire, would bring a new perspective. Caius might start to see the world a bit differently through her eyes, gradually developing a deeper emotional connection as they face various challenges together in the vampire society.
Over time, their relationship would also be shaped by the reactions of their respective families and communities. Caius' vampire coven in Twilight might be suspicious of his connection to a Mikaelson - related soulmate. And the Mikaelson family might have their own reservations. But as they stand up for each other against these external pressures, their bond becomes unbreakable.
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.
If Zoey were to lose her memory and Logan was the only one who could help her remember, that would be a unique development. He could take her to all the places they used to go together and tell her stories about their past. This way, Zoey would see Logan in a whole new way, not just as the guy she knew casually, but as someone who really cares about her and has a deep connection with her.