One possible reason could be the trauma he has faced in his life. For example, the loss of his parents at a young age and the constant feeling of being an outcast in the village. These past experiences might resurface in his dreams as nightmares. Another reason could be the pressure he feels as he grows older, like the weight of his responsibilities as a ninja and his role in protecting the village. Also, the fear of losing his friends, who have become like family to him, might haunt him in his sleep.
I recommend two novels, Dancing Phoenix in the Nine Heavens: It's Not Easy to Be a Princess in Time Travel and Happy Marriage. The former female protagonist had a bad reputation after she transmigrated into a princess, causing her nightmares. The latter female protagonist had nightmares for three days, causing her to transmigrate into a person in her dreams. I hope you like this fairy's recommendation. Muah ~😗
They can also expand our view of their relationship. Fanfictions about nightmares might introduce new scenarios where their relationship is tested in ways not seen in the books. For example, a nightmare where Edward has to choose between Bella and his vampire family can show how complex their relationship is, as it involves different loyalties.
One common theme is the exploration of Bella's fear of losing Edward. In the Twilight series, their relationship is intense, and in fanfictions about nightmares, this fear might be magnified. For example, she could dream that Edward leaves her for his vampire family's sake. Another theme is the danger that Edward's vampire nature poses in her nightmares. Maybe she dreams of him losing control and hurting her, which is a contrast to his usually protective nature in the original story.
Bella's nightmares can also show her struggle with her new vampire identity. If she has a nightmare about not being able to control her thirst, it ties into her character's journey of self - discovery as a vampire. She was always a bit of an outsider as a human, and now as a vampire, she has new challenges to face. Carlisle's nightmares about his past sins are very much in line with his character. He is a morally upright vampire, constantly trying to atone for his past actions. These nightmares are a reminder of the internal conflict he has between his past and his present self, and how he is always striving to be better.
I'm not sure if there is a specific 'Twilight' fanfic about that. Fanfic topics can be extremely diverse, but such a topic might be considered inappropriate or not in line with the general tone of the 'Twilight' series. The official 'Twilight' works mainly focus on the relationship between Bella and Edward, the vampire - werewolf conflict, etc.
Perhaps in the fanfiction world, Bella caught a mysterious disease that's specific to that fictional universe. It could also be that she overexerted herself, like using her special powers too much if she has any in the story. Stress can also be a factor. If she's been through a lot of emotional turmoil, like a fight with a loved one or facing a big threat, her body might react with a fever.
Well, it might signify her uniqueness. In the Twilight universe, Bella is already different as she is the object of Edward's affection and has a special connection to the vampires. Wings could be an added feature that sets her even further apart. It could also be related to some sort of power or ability she gains. Maybe she can use the wings to fly to places quickly, which would be useful in both the human and vampire worlds, especially when she is in danger or needs to reach someone fast.
It could be the Volturi. Since the baby is a vampire - human hybrid, the Volturi, who are very strict about vampire laws, might see the baby as a violation of their rules and try to eliminate it.
Well, one possibility could be that the baby has some special powers right from the start. Maybe it can communicate with the vampires in a unique way that no other vampire - human hybrid has before.