Edward would be an obvious choice as a main supporting character. He would be there for Bella, sharing in her joys and fears, and helping with all the preparations for the twins. Another could be Alice. She might use her ability to foresee things to help Bella prepare for any challenges during the pregnancy.
Hermione would definitely be a major supporting character. She'd be using her vast knowledge to help Harry throughout the pregnancy.
Alice could be a supporting character. She might have visions about Bella's pregnancy and the future of the baby. Jacob could also be involved, perhaps initially being angry or concerned about Bella's situation with Aro. Carlisle could be another one, as his medical knowledge as a vampire doctor could be useful in dealing with Bella's pregnancy.
Bella and Elena are the stars. They might have a mentor figure in the fanfiction, someone who helps them understand their unique situation, whether it's related to their powers (if there are any in the story) or their family secrets. And there could be antagonists too, like a jealous relative or a person who wants to use their special connection for their own gain.
One idea could be that Bella is facing a lot of mixed emotions. She's excited about the twins but also worried about how to handle two babies at once. Maybe she starts preparing the nursery early, painting it in soft pastel colors and filling it with cute baby stuff.
You could begin by setting the scene in a really emotional moment. Maybe Bella is having a conversation with a close friend and she breaks down crying because she has a feeling she's carrying twins but she's not sure how to handle it. This creates an air of mystery and makes the reader want to know more.
If it's a more modern - day or non - supernatural setting, Bella's parents could be main characters. They might be overjoyed at the birth of their grandchildren and be actively involved in their upbringing. They could provide financial support or just be there for emotional support for Bella, especially during the tough early days of raising twins.
The main characters are clearly Bella and Quil. Bella is likely to be the one carrying the baby, and Quil is the father - or at least an important figure in the pregnancy storyline.
The main characters are Embry and Bella. Embry is likely a werewolf in this fanfiction context, and Bella is the one who is pregnant.
The main characters are Bella and Aro. Bella is often a human - turned - vampire or has some special connection to the vampire world. Aro is a powerful vampire with his own set of abilities and a complex personality.
There could also be new characters introduced in the fanfiction. Maybe a doctor who specializes in the unique situation of a human - werewolf pregnancy, or a friend who has been through a similar experience and can offer advice to Jacob and Bella. But overall, Jacob and Bella remain at the heart of the story.