A jealous vampire or witch could also be an antagonist. Maybe a vampire who was once in love with Damon or a witch who has a grudge against Elena. They could try to harm the baby out of spite or envy. They might use dark magic or their vampire abilities to try and separate the baby from its parents or cause harm to it in other ways.
Another group could be the vampire hunters. If they find out about the baby, they would likely see it as an abomination and try to kill it. They could be a constant threat, forcing Damon and Elena to keep moving and hiding. They might also team up with other supernatural enemies of Damon and Elena to try and take down the baby.
Well, perhaps the story could revolve around how their relationship changes with the new addition. They might struggle to balance their love for each other and their newfound parental responsibilities. For example, Damon's wild side might need to be tamed a bit more as he becomes a father, and Elena has to deal with the stress of being a mother in a world full of danger.
One possible plot could be that they face many challenges in raising the baby. Maybe there are threats from the supernatural world that target their child. Damon, with his protective nature, would be on high alert all the time, while Elena uses her compassion to raise the baby in a loving environment.
Another possible antagonist could be a powerful vampire hunter. He sees Damon as a threat and Elena as a potential target or a way to get to Damon. When Damon and Elena meet, this hunter is already watching them. He plans to use their new relationship against them, making their first meeting the start of a lot of trouble for both of them.
The other vampire clans might be antagonists. They could see the baby as a threat to the vampire hierarchy or fear that the baby will have powers that could disrupt the balance among vampires. For example, the Originals' siblings who don't trust Kol might try to stop Elena from having the baby.
The other vampires in Mystic Falls who are against the idea of a hybrid - vampire baby could be antagonists. They might see it as a threat to their way of life.
It could be an old vampire enemy of Damon's. Maybe someone from his past who has a vendetta against him and sees the baby as a way to get revenge. This antagonist could try to kidnap the baby or cause trouble in their lives.
The Foot Clan could be a main antagonist. They might see the baby sister as a way to get to the turtles, either by kidnapping her or using her to blackmail them. Another could be Shredder himself if he returns. He would surely not pass up the opportunity to use the baby sister against the turtles. And Baxter Stockman could also be an antagonist. He might be interested in using the baby sister for some of his mad - scientist experiments.
There could be a rogue Saiyan who has different ideals from Vegeta. This Saiyan might think that the baby should be raised in a more traditional, warrior - like way and try to take the baby from Bulma and Vegeta. Or, an evil scientist who wants to experiment on the baby's unique Saiyan - human DNA could be the main antagonist, creating all kinds of challenges for Vegeta and Bulma to protect their child.
It could be some rogue shinigamis who oppose the idea of rukia and ichigo having a child together. They might see the baby as a threat to the existing power structure in the Soul Society.
In the 'Harry and Daphne baby fanfiction', it could be a new dark wizard who rises to power. This wizard might believe that the baby has some sort of power that could stop him from achieving his goals. So, he makes it his mission to eliminate the baby. Also, there could be some magical creatures that are being controlled by a dark force and are sent to attack Harry, Daphne and their baby because they sense the baby's potential power.