Another possibility is a group or organization that has different ideals. If Thalia and Luke are part of a group that values something different from them, that group could oppose their decision to have a child and become the main antagonists, trying to stop the pregnancy or take the baby away once it's born.
One possible plot could be that they face a lot of challenges. Maybe they are in a world full of danger and they have to protect their unborn child. They might need to find a safe place to stay, away from their enemies. Thalia could be using her skills to keep them safe while Luke tries to figure out how to provide for their future family.
Sith cults could also be antagonists. They might sense the power of the unborn child in the Force and want to turn it to the dark side. They could launch attacks on Luke and Mara to either kidnap Mara or influence the unborn child through dark side magic or the Force.
The Capitol could still be the main antagonists. They might see Katniss's pregnancy as a threat, either because they fear the child could grow up to be a symbol of resistance like Katniss or just because they want to maintain control over her. They could try to harm her or the baby in some way.
In some fanfictions, it could be an ancient and powerful Titan who has escaped from Tartarus and is seeking revenge on the demigods. This Titan might see Percy and Thalia as the main obstacles in his path to domination.
The Emperor could still be a major antagonist. Even though Anakin turned on him in the end, his influence might still be felt. He could send his agents to try and find baby Luke to eliminate any potential threats to his power.
The main antagonists could be some of the more traditional pure - blood families who look down on Ron and Luna's relationship. They might see Luna as an oddity and Ron as not being from a high - enough status family. They could try to interfere with the pregnancy, either by spreading false rumors or trying to separate the couple.
It could be the traditional enemies from Naruto's world, like the Akatsuki. They might see the union of Naruto and Rias and their unborn child as a threat to their plans for world domination. They could try to kidnap Rias to use her as a bargaining chip against Naruto or to study the unique nature of their child.
The old enemies of Percy and Annabeth like the Titans or some of the more vengeful monsters could be the main antagonists. They might see the unborn child as a threat or an opportunity to get back at Percy and Annabeth.
Their friends could also be antagonists in a sense. For example, if there's a friend who has always had a crush on Freddie and is jealous of Sam. This friend could spread rumors about Sam being a bad influence or try to convince Freddie that he's making a huge mistake by staying with Sam and having a baby. Another scenario could be that their friends don't take the situation seriously and keep pressuring them to act like they used to, which makes it hard for Freddie and Sam to focus on the pregnancy.
It could be the jealous servants in the Beast's castle. They might not like Casey in the first place, and her pregnancy could make them fear that she will gain even more power and influence. So, they could try to sabotage her pregnancy in secret ways.