The Mikaelson family, especially Klaus, could be antagonists. He might see the pregnancy as a threat to his family's power or reputation. He could try to stop Stefan and Rebekah from being together and having the baby.
Some of Stefan's vampire friends who think that having a baby with a vampire - witch hybrid like Rebekah is a bad idea. They might fear that the child could be too powerful and cause chaos in the vampire world. So they could try to interfere and separate Stefan and Rebekah.
One possible plot could be that Stefan and Rebekah find themselves in an unexpected situation where they are about to have a baby. There could be a lot of drama as they deal with their families' reactions. Maybe the Mikaelsons are initially against it, while Stefan's friends are also shocked. They would have to navigate through their own relationship issues while preparing for the new life coming.
One possible plot twist could be that the pregnancy is a result of a magical spell gone wrong. Maybe a witch with a vendetta against them cast a spell that made Rebekah pregnant with Stefan's child as a way to disrupt their lives. Another could be that it's a false pregnancy caused by some dark entity preying on their fears of starting a family. This would add a lot of drama as they first deal with the shock of the pregnancy and then the realization that it might not be real.
Rebekah's family has a long history of complex relationships. Her mother, if still around, might see the pregnancy as a way to strengthen their family's position in the supernatural world. However, some of her siblings like Kol might be jealous or feel left out. On Stefan's side, his vampire nature might make his family worry about the implications of having a half - vampire child. They would be concerned about how to raise a child in their world of danger and secrets.
In this fanfiction, there might be some werewolves who view the baby as an abomination since it's part vampire. They could try to harm the baby, which would put Bella and Stefan in a difficult position to defend their child.
Character development is a big characteristic. As the story progresses, both Stefan and Rebekah usually change. Stefan might become more lenient towards Rebekah's flaws, and Rebekah might start to develop a more empathetic side due to Stefan's influence. This transformation of their characters within the fanfiction is what makes it so appealing to many readers. Fanfictions also often explore their pasts, like Stefan's history with the Salvatore family and Rebekah's long - standing issues with her own family, the Originals, and how these pasts shape their relationship in the present.
A good alternative would be to explore their family relationships. Rebekah has her complex family history with her siblings, and Stefan also has his own family - related issues. Writing about how they might interact regarding family matters, like Rebekah giving Stefan advice on dealing with his brother Damon, could be a really engaging fanfiction concept.
It could be Marcel. He has a long - standing relationship with the Mikaelson family, and his connection with Rebekah could deepen during her pregnancy. He might be the one to support her through the difficult times.
There could be a new vampire hunter in town. This hunter could target Stefan and Elena because of their relationship. He might believe that their union will create a more powerful vampire line and wants to stop it before it's too late.
It could be that an old enemy of Stefan's family decides to target Bella as a way to get back at Stefan. This enemy might be another vampire clan or a supernatural being with a long - standing grudge. They would do everything in their power to disrupt the relationship between Bella and Stefan, causing all sorts of problems and conflicts for the two of them in the fanfiction.
One possible plot twist could be that an old enemy resurfaces during the wedding. Maybe a vampire from their past who has a vendetta against either Stefan or Rebekah shows up to disrupt the ceremony. This could lead to a fight scene where Stefan and Rebekah have to put their newfound union aside for a moment to deal with the threat. Another plot twist could be a family member's objection. For example, Klaus might object to the marriage as he has his own plans for Rebekah and doesn't want her to be tied to Stefan.