The baby itself could be a sort of main character in a way. It might have its own unique personality, even as a baby. And it could be the focus of the story as the characters try to protect it, raise it, and figure out what kind of future it will have in the Twilight world.
In Naruto fanfiction, characters like Jiraiya could also be a 'baby daddy'. He was known for his womanizing ways in the series, so it's not hard to fathom a situation where he has a child. Then there's Itachi. Although his life was full of tragedy and secrecy, a fanfiction could explore a scenario where he has a child before his untimely death. And Kiba could be a character as well. His close bond with his dog Akamaru and his brash personality could lead to a situation where he becomes a 'baby daddy' in a creative fanfiction.
Obviously, the mother is a main character. She's the one driving the search for the secret daddy. Then there's the baby, who is sort of the reason for the whole story. And of course, the potential fathers. There could be one main potential father who is the focus of the search, or there could be a few guys who are all suspects in a way.
Well, one possible plot could be about the challenges of raising a child in the world of Twilight. Maybe the baby daddy is a vampire and they have to deal with the baby having some special abilities or needs because of its vampire heritage.
Iruka sensei is another possibility. He is kind and nurturing, having already shown his care for Naruto when he was a student. As a baby daddy, he would be patient and dedicated to raising his child with good values and a love for learning.
Edward and Bella are likely to be main characters. Since they are central to the 'Twilight' story, in a fanfiction about a living will, they could be involved in making decisions regarding the living will, perhaps related to their future together or the safety of their family.
There could also be side characters like the baby daddy's friends or family members. His best friend might be trying to convince him to either step up or run away from his responsibilities. Or his mother could be a traditional figure who has very specific ideas about how he should handle the situation, adding more conflict to the story.
Edward Cullen is one of the most common baby daddies in Twilight baby daddy fanfiction. Since he's the main male lead in the original series and has a complex relationship with Bella, it's natural that many fanfiction writers explore the idea of him as a father. Jacob Black is also a popular choice. His relationship with Bella was also significant, and in some fanfictions, he could end up being the baby daddy in an alternate storyline. Another character who might be the baby daddy in some fanfictions is Aro from the Volturi. His powerful and mysterious nature makes for an interesting take on being a father in the Twilight - inspired stories.
Another possibility is that the fanfiction could introduce new characters who play a major role in the 'coming home' story. They could be new vampires or humans who have a connection to the original Twilight characters. For instance, a new vampire who was turned while Bella was away and now she has to deal with this new presence when she comes home, and this new character could bring a whole new set of conflicts and relationships into the story.
Ben and Josh are the central figures. Ben could be portrayed as the more impulsive of the two. He might make hasty decisions that often get them into funny or tricky situations. Josh is probably the one who tries to keep things in order. He's the one who thinks about the consequences and tries to guide Ben in a more responsible direction. Their relationship is what drives the fanfiction forward.
Riley and Danny are the main characters. Riley might be a character with a mysterious past that slowly unfolds throughout the story. Danny may have a strong family background that influences his actions towards the baby. And the mother of the baby is also a crucial character as her relationship with both Riley and Danny drives the story forward.