It could be Edward and Jacob. Since they both have strong connections to Bella in the original 'Twilight' series, it would be an interesting twist to have them form a triad with her in fanfiction.
One interesting aspect could be the new relationship dynamics. In a triad, Bella Swan's character might experience more complex emotions and interactions compared to the original story. For example, she could have to balance the affections of two different characters, which would lead to a whole new set of storylines and character developments.
It could be a character named Jake. He's a very competitive Bakugan brawler who sees Bella as a threat because she's quickly rising in the ranks. He's always trying to one - up her in battles and spread rumors about her to make other brawlers not trust her.
Perhaps it's a human with a very strong psychic ability. He is drawn to Bella because he can sense her connection to the supernatural. Their relationship could be based on mutual understanding of the secrets they both hold, and Bella might find his normalcy in some ways refreshing compared to her previous relationships with vampires and werewolves.
Edward could be one of her allies. He could use his vampire abilities such as super - speed and strength to assist Bella in dangerous situations. His centuries - old knowledge could also prove invaluable in understanding the motives and actions of the CIA's enemies. Another ally could be Jacob. His shape - shifting abilities would give the team an extra edge in combat or in covert operations. He could also use his connection to the werewolf pack to gather information from areas that the CIA might not have easy access to.
Maybe it's Loki. Loki is known for his mischief and cunning. His complex personality would add a lot of drama to the relationship with Bella. He could bring a whole new level of chaos to the Twilight universe.
It could be a new vampire. Maybe a vampire from a different coven who comes to Forks and is immediately attracted to Bella's unique scent and her strong - willed personality.
Edward can be really interesting. His relationship with Bella takes on a new dimension when she's a siren. He has to grapple with his love for her while also being cautious of her new powers. Another character could be Jacob. His loyalty to Bella might be tested as she becomes something different, and he might have his own mixed feelings about her siren side.
Alice Cullen can be a very popular character. Since she can see the future, in a 'reborn as bella swan' scenario, her interactions with Bella could be really different. Maybe she tries to guide Bella in a new way based on her visions. Also, Carlisle Cullen is popular. His father - figure role towards Bella might be explored further in these fanfictions.
Perhaps a human character who has an unhealthy obsession with Bella. Maybe someone who has always been in the background, like a shy classmate who has developed an extreme fixation on her and is now stalking her to be closer to her in a very wrong way.
It could be a jealous vampire who doesn't like Bella's new - found power. Maybe this vampire wants to be the most powerful being in the area and sees Bella as a threat.