It depends on the specific fanfiction. Some fanfictions focus more on action or other plot elements rather than fully developing their relationship. However, in the ones that do, it can be a really interesting and complex relationship with a lot of emotional depth.
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.
In fanfiction, their relationship might develop through a series of adventures. Suppose they go on a quest to find a rare artifact that has the power to either save or destroy the vampire world. Along the way, they face numerous challenges like dangerous terrains and other powerful vampires. During these hardships, they start to care for each other more. Peter might show his protective side towards Bella, and Bella might be the one to give Peter hope and a new perspective on life. This would strengthen their bond.
Bella Swan's character could be developed by having her explore different aspects of herself. In the context of Greek gods, she might discover a more passionate side of herself, like the passion associated with the Greek concept of eros. This could make her more confident and change her relationships. She could also be developed through her experiences with the Greek gods' powers. If she gets a taste of Athena's wisdom, she could use it to solve problems in Forks that she couldn't before. This would make her a more respected and important figure, not just among the Twilight characters but also among the Greek gods she encounters.
At first, their relationship might become strained. Bella's sister would have to keep her new identity a secret from Bella, which could create distance. But once Bella finds out, they could become closer as Bella tries to understand what her sister is going through. Bella might be more protective of her sister now that she's in a more dangerous world.
To develop Bella in this fanfiction, she could be given a mentor. Maybe an old witch who has been watching over her. With the mentor's help, she can learn about the history of doppelgangers in the witch world and how to control her powers. She can also struggle with the morality of using her powers, like whether she should use them to get revenge on those who have wronged her in the past.
One way is to explore the backstory of Petrova more deeply. If Bella is a fan, then showing what makes Petrova so fascinating to her through detailed backstories would be great.
One way the story could be developed is by having Bella be a secret Mikaelson fan in the Twilight universe. Maybe she discovers a hidden book about the Mikaelsons in the Forks library. This could lead to her having strange dreams about them and it might even influence her relationship with Edward. For example, she could start comparing Edward's vampire nature to that of the Mikaelsons.
Simon's character can be developed by exploring his backstory. If he has a troubled past, it can be revealed gradually as he gets closer to Bella. Also, his relationship with Bella can make him more empathetic or more guarded depending on how their interactions play out. For example, if Bella is initially suspicious of him, he might become more defensive in his behavior.
Well, in fanfiction, Bella's character could be developed in a way that she explores her new vampire nature more deeply. She might be more curious about her powers and how they can be used in relation to Jacob. For example, she could discover that her vampire senses can help her understand Jacob's wolf side better. Jacob could be developed to be more empathetic towards Bella's vampire life. He might try to find common ground between his wolf pack and Bella's vampire world, which is a big shift from his initial hostility in the original story.
In fanfiction, Bella Swan as the mate of a wolf often has a special connection that goes beyond the normal. It could be a psychic link or an unbreakable bond. This makes their relationship different from typical human relationships and adds an element of mystery and magic.