To develop the story, start with their reaction when they find out about the pregnancy. Are they shocked, happy, or scared? Then, think about how their relationship changes. Maybe they become more protective of each other.
Don't forget about the secondary characters. Their friends and family can play important roles. Maybe Felix's best friend gives him some bad advice about how to deal with a pregnant Bella, which causes some misunderstandings between them. Or Bella's mother could come to stay with them to help out, but they end up having different opinions on how to raise the baby. This can all make for an interesting fanfiction story.
One idea could be that Felix and Bella face challenges during the pregnancy. Maybe they have to deal with family reactions that are not all positive. Some family members might be worried about their financial situation or their young age.
In an alternate universe, their relationship could start as a friendship based on mutual respect. Felix might admire Bella's courage in the face of danger, and Bella could be intrigued by Felix's strength and loyalty to the Volturi. This friendship could gradually turn into a deeper emotional connection as they face various challenges together.
For Bella, you could show her growing in confidence as she deals with the pregnancy. At first, she might be scared and unsure, but as time goes on, she could become more assertive in making decisions for herself and her unborn child. For example, she could start taking charge of her prenatal care and making plans for the future, which shows her strength.
She could also develop more compassion. Knowing she's bringing a new life into the world, she might start to see things from a different perspective, especially when it comes to the relationships within the Cullen family and their views on her pregnancy. This could lead to her trying to bridge gaps and bring the family closer together.
Bella could become more independent. With Paul leaving, she has no choice but to rely on herself. She might become more assertive in making decisions for herself and her unborn child.
Well, Emmett might become more serious and responsible. He has to think about not just Bella but also the unborn child. He could start making plans for their future as a family, which would change the dynamic of their relationship. For instance, he might start thinking about where they will live and how they will raise the child. Bella, in turn, would see this new side of Emmett and fall in love with him even more. Also, they might have some small arguments over things like Bella's diet during pregnancy (since it's a special situation with a half - vampire baby), but these arguments would only strengthen their relationship in the long run as they learn to compromise.
Bella might develop into a more confident and motherly figure. Since she's been through pregnancy before, she would be more prepared this time, but still face new challenges. Edward, on the other hand, could become more conflicted. He loves Bella and their unborn child, but is constantly worried about the dangers that lurk. Jacob, if he's still in the picture, might be a bit jealous at first but then come around to support them. He could also be a protector of the family in his own way.
As the pregnancy progresses, Bella often experiences changes that are both physical and mental. Physically, she might have a much tougher time than a normal human pregnancy because of the vampire - human mix. Mentally, she has to deal with the fear of the unknown. The story might also focus on how Edward and Bella's relationship is affected. They could grow closer as they face this new challenge together, or there could be some strains due to their different views on the pregnancy.
Often, their interactions are full of tension. Since the Volturi and the Cullens have a complex relationship, Felix and Bella are on opposite sides at first. However, in some stories, Bella's kindness towards Felix, despite his fearsome reputation, can start to change his perception of her. He may start to see her as an individual rather than just a part of the Cullen clan, leading to more positive interactions.
The fanfiction could also focus on Bella's character development as Felix's sister. She might have her own dreams and goals, but at the same time, she always puts Felix first. For instance, if Felix gets into trouble with some other characters, Bella could use her charm or intelligence to get him out of it. It could be a story about loyalty, family, and growing up together in a fictional universe.