One possible scenario could be that there were complications during the birth that couldn't be overcome. Maybe there was excessive bleeding or an undetected health issue that led to her death.
One possible plot could be that the story focuses on the grief of those left behind, like her partner. They might struggle with the sudden loss and have to figure out how to take care of the newborn without her. Another plot could involve the backstory leading to her death, perhaps there were complications during the birth that no one saw coming. And it could also show the impact on the community around her, how they come together to support the family in this difficult time.
Perhaps the Cullen family has to deal with the shockwaves of Bella's death. They may face challenges from the vampire community as Renesmee is a unique being. Some vampires might see her as a threat or an abomination. The family would then have to protect Renesmee at all costs while also mourning Bella.
In certain 'Bella dies in Edward's arms' fanfictions, it might be related to time - travel elements. Imagine a situation where Bella travels back in time to change something for the better in Edward's past, but in doing so, she disrupts the timeline in a way that causes her own death. When she returns to the present (or what was the present before the time - travel), she is already dying and Edward holds her as she passes away. Another idea could be that Bella's human frailty, combined with the dangerous vampire world she's a part of, leads to her being in a situation where she can't be saved, and Edward has to watch her die in his arms.
The baby, of course. Growing up without a mother would be a huge part of the story. The father would also be deeply affected, having to raise the child alone and deal with the grief of losing his partner.
Her partner could be a main character. They would be deeply affected by her death and would likely have to take on a lot of responsibilities, especially in terms of raising the baby. The baby itself could also be considered a main character in a sense, as the story could follow its growth and development in the absence of its mother.
Another scenario could be that Gohan contracts a deadly and incurable disease in the fanfiction world. Despite the efforts of his family and friends, including the use of any available advanced technology or magic, he succumbs to the illness, leaving everyone in mourning.
One possible scenario could be that Luffy sacrifices himself to save his crew during a major battle against an extremely powerful enemy. He might use all his strength and energy to protect them, and in the end, it's too much for his body to handle.
One possible plot could be that the story focuses on the emotional turmoil of those around Sakura. Her friends and family might be in shock and disbelief as they were expecting a joyous birth but instead are faced with her death. They could struggle with guilt, thinking if there was something they could have done to prevent it.
One common theme could be sacrifice. Sakura's death during childbirth can symbolize the ultimate sacrifice a mother makes. Another theme might be the impact on the other characters. For example, it could show how the father copes with the loss and the responsibility of raising the child alone. Also, it might explore themes of grief and how the community around them reacts to such a tragic event.
Some Bella giving birth fanfiction stories may explore different scenarios. Maybe it's a difficult birth that makes everyone worry about Bella's life. This kind of story often builds a lot of suspense. The author might describe in detail the medical procedures and the efforts of the doctors (if there are any in the fanfiction world) to save both Bella and the baby.