It could be that Elijah's own family members play the role of antagonists at first. Maybe some of them don't approve of Bella, thinking she is not suitable for Elijah. This could create a lot of internal conflict within the family and also between Bella and Elijah as they try to overcome the family's objections.
Maybe there are some traditionalists in their world who don't approve of their relationship or the pregnancy. These people could be the antagonists, constantly trying to break them apart or cause problems for them.
A potential main character could be Bella's best friend. This friend could provide support and a different perspective as Bella navigates her new identity. There might also be a mentor figure for Bella, someone who knows a lot about Elijah's world and can guide her. This could be an old friend of Elijah's or a wise elder in their community.
One common antagonist could be Klaus. He has his own motives and often clashes with Elijah. His presence can create a lot of tension between Elijah and Elena as well, since he might target Elena to get to Elijah or vice versa.
One possible plot could be that Bella and Elijah face challenges from the vampire and werewolf communities. Their relationship might be seen as an oddity or a threat. For example, other vampires could be jealous of Elijah's relationship with Bella, or werewolves might see Bella as a target because of her connection to a powerful vampire like Elijah.
The main characters are clearly Elijah and Bella. Since they are married in this fanfiction, they are likely at the center of the story, with their relationship being the main focus.
Obviously, Elijah and Bella are the main characters. Elijah is likely to be the hero or the savior in this story. He probably has some special abilities or qualities that enable him to save Bella.
Sometimes, other werewolves who don't accept Bella as Jacob's mate could be the antagonists. They might think that a human - werewolf relationship goes against their pack's rules or beliefs.
Often, other vampires or supernatural creatures could be the main antagonists. For example, vampires who are jealous of Damon's newfound mate might try to drive a wedge between them. Maybe there are some werewolves who see Bella as a threat to their territory because of her relationship with Damon.
Often, the Volturi can be antagonists. They have strict rules about vampire relationships and activities. If Bella and Jasper's relationship goes against their laws, the Volturi would oppose it. They might see it as a disruption to the vampire order they uphold.
The Originals' enemies, like the werewolves, could be antagonists. They might see the baby as a symbol of the vampires' growth and power and try to harm it. Also, some of the other vampire factions who are against the Mikaelsons could target the baby to get back at Elijah and Katherine.