The relationship could be developed through a mutual goal of protecting someone or something. For example, they could both be on a mission to protect a group of humans from other dangerous vampires. Working together towards this goal would build trust and understanding between them, and as they face various challenges along the way, their love for each other would grow in a more complex and interesting way.
One popular storyline could be about Rosalie's soulmate helping her overcome her past traumas. Since Rosalie has had a difficult past, her soulmate might be the one to bring out the softer side of her and help her heal. For example, they could go on adventures together that force her to face her fears in a new light.
They could start by having common goals. For example, both might be interested in protecting a certain human from the darker elements of the supernatural world. This shared objective can bring them together and make them interact more, gradually building a relationship.
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.
There are ones where they switch roles. Rosalie takes on Edward's more brooding and reserved nature while Edward becomes more outgoing like Rosalie. It's an interesting take on their personalities.
The twin could create jealousy in Rosalie if Emmett shows too much interest in the new character. Since Rosalie is very protective of her relationship with Emmett, any threat, even if it's just a perceived one from her twin, could cause a rift between her and Emmett. On the other hand, it could also strengthen their bond if they work through it together.
One great Rosalie Hale OC fanfiction is 'The Newcomer'. It has an interesting OC who shakes up the dynamics in the world of Rosalie. The OC has a unique backstory that ties in well with the Twilight universe.
The fanfiction community often portrays Rosalie Hale's pregnancy as a very complex and emotional journey. Since she's a vampire, there are many challenges and differences compared to a human pregnancy.
They could interact through a magical accident. For instance, a spell gone wrong could open a portal that brings Rosalie into Harry's world. Then they'd have to figure out how to get her back while also dealing with the threats in the wizarding world like Voldemort.
Edward Cullen is often a popular character in these fanfictions. Since he has a close relationship with Bella in the original story, his role can be further explored in relation to Rosalie and Bella's relationship. Another one could be Alice Cullen. Her ability to see the future can add an interesting dynamic, especially when it comes to predicting things that might affect Rosalie and Bella's friendship or any adventures they might go on.
I really liked 'Edward and Rosalie: Unseen Moments'. This fanfiction focuses on their shared experiences in the Cullen household. It gives a fresh perspective on their friendship and how they deal with the challenges of being vampires together. There are some really interesting dialogues that bring out their personalities.