It could be demons. Demons are often the main antagonists in the Supernatural universe, and they might target Bella because she is related to the Winchesters. They could try to use her as a bargaining chip or a way to get to the brothers.
Angels could also be antagonists in this fanfic. Some angels in the Supernatural series have their own agendas and might not like the idea of Bella being part of the Winchester family. They could try to manipulate situations to get rid of her or use her to achieve their own goals that might not be in line with what Dean and Sam want for their sister.
In some fanfics, there could be a plotline centered around Bella's love life in the context of being Dean and Sam's sister. Maybe she falls for another hunter, and the brothers have to approve or disapprove. There could also be a plot where Bella is haunted by a past event that only she remembers, and she has to rely on her brothers to help her figure out what really happened and how to move on.
Another possibility is that there is a mysterious figure who has some sort of vendetta against the family. This figure could be causing strange and unexplained events to occur around the baby, like making the baby sick in a way that doctors can't figure out at first. This would then be a source of conflict as Bella and Sam try to protect their baby from this unknown threat.
Obviously, Dean and Sam are main characters. They are the protective older brothers. And then there's the baby sister, who is likely a central figure as well, with her own personality and role in the fanfiction. There might also be other recurring characters from the 'Supernatural' universe like Castiel who could play important parts in the story.
The main characters are Harry, who is the focus as Sam and Dean's sister. Sam, who is good at research and understanding the deeper aspects of the supernatural. And Dean, who is more hands - on and ready to fight. Their different personalities and how they interact with Harry form the core of the story in this fanfiction.
Well, it's hard to say for sure without reading the fanfic. But Castiel is a very common choice as he has been so closely associated with Sam and Dean in the original series. There could also be some lesser - known angels that the author has created specifically for this fanfic. Maybe an angel that represents a different aspect of angelic society, like an angel in charge of angelic education or something, who takes a special interest in Sam and Dean.
In some cases, a human antagonist could work. For example, a jealous classmate who is envious of Bella's sister's relationship with a popular boy. This could lead to a lot of petty but still impactful drama at school and in the community.
Yes, there might be. The 'Supernatural' world of Sam and Dean and the 'Harry Potter' universe are both very popular, and fanfiction often likes to cross over different fandoms in creative ways. Some fans might have come up with the idea of making Harry Potter be related to Sam and Dean as their sister in a fan - created story.
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.
One idea could be that Bella discovers she has special powers because of her relation to Sam and Dean. Maybe she can communicate with spirits in a unique way that helps them on their hunts.
The little sister herself can be a great character. She often brings a fresh perspective to Sam and Dean's world. She might be brave but also have a vulnerability that makes her relatable. For example, she could be hesitant about the hunting life at first but then grow into a strong fighter.