Esme Cullen, the matriarch of the Cullen family, would surely be part of the story. She would be very nurturing towards Bella during her pregnancy. Jasper Cullen might also be included, perhaps using his power to influence the emotions of those around Bella to keep her calm during the pregnancy. And of course, there could be other minor characters from the Twilight universe popping up in the fanfic.
Bella is the main focus, of course. If it's in a crossover fanfiction, other characters from different fictional worlds could be included. But in a more traditional setting, characters like Rosalie might be there. Rosalie, with her own experiences related to motherhood or the lack thereof, could have a complex relationship with Bella during her pregnancy. Alice might also be a common character, using her ability to foresee things related to the pregnancy, like whether it will be a smooth process or if there are any upcoming challenges.
In some fanfics, there could be a plot where Bella's pregnancy is a threat to the vampire - human truce. This would create a lot of tension as the vampire community has to decide how to deal with this new situation. Also, a plot could revolve around Bella's dreams and premonitions during her pregnancy, which could give hints about the future of her unborn child.
Edward is an obvious choice. As the father of the baby, his role is crucial. He can be portrayed as overprotective, or struggling to understand the new situation. Jacob could also be included. His relationship with Bella is complex, and his reaction to her pregnancy could add a lot of drama. Then there's Carlisle. As a doctor - like figure in the vampire world, he can offer medical advice and support during the pregnancy.
Well, obviously Bella herself. Then there could be the father of the baby, if it's not a mystery. There might also be her friends or family members. They could play different roles, like being supportive or having doubts about the situation. And perhaps there are some medical characters, like a doctor or a healer if it's in a more fantasy - based setting, who are involved in taking care of her during the pregnancy.
The main characters would be Naruto, who brings his ninja - like abilities and unique personality from his own series. Then there are the Cullens - Edward, Carlisle, Esme, etc. - each with their own vampire abilities and family dynamics. And of course, Bella, who is a central figure in the Twilight series. In this fanfiction, they are all likely to be involved in some sort of story arc together, probably with Naruto having some sort of impact on the relationships and situations involving the Cullens and Bella.
Another theme is the transformation of Bella. Physically, she is changing, but also her role in the story. She becomes more nurturing and protective even before the baby is born. This is shown in different ways in various fanfics, like how she starts to decorate the nursery early on.
The relationship between Bella and Edward (assuming it's a Twilight - related fanfic) also changes. They have to navigate their love in a new context of impending parenthood. This can include things like Edward being over - protective or Bella worrying about how their relationship will be affected.
Obviously, Bella and Sam are the main characters. Bella is the one who is pregnant, so she will likely be dealing with a lot of emotions and physical changes throughout the story.
Obviously, Edward Cullen and Bella Swan are the most important. Edward, the vampire, is central to the story as Bella's groom. Bella, the human - turned - vampire - later in the original series, is the bride. Then there's Alice Cullen. She is often a key character as she loves to plan events and would be all over the wedding preparations. Jasper Cullen might also be important as he could help keep the emotions in check during the big day. And of course, Charlie Swan, Bella's father, his presence or his thoughts about the wedding could be a significant part of the fanfiction.
One common theme is the exploration of their eternal love. Writers often expand on how their love endures through different challenges. Another theme is the power dynamics, especially considering Edward's vampire nature and Bella's human - turned - vampire state in some stories.