Maybe the Fates themselves could be antagonists in a way. If they have a different plan for Percy and Aphrodite, they could try to interfere with the pregnancy. Their actions could be subtle, like causing small misfortunes or trying to make Percy doubt his relationship with Aphrodite. Since they control the destinies, they might not want this unplanned union to succeed and would use their powers to put obstacles in the couple's way.
The main antagonists could be some of the other gods who are jealous of Aphrodite's special situation with Percy. For example, Hera might not like the idea of Aphrodite having a child with a demigod, as it goes against the normal order of things in the godly realm.
The children of Ares might also act as antagonists. They are often at odds with the followers of Aphrodite in the Percy Jackson universe. They might see Percy being the champion of Aphrodite as a threat to their own power and influence. So, they could constantly try to pick fights with Percy, both physically and by trying to turn other demigods against him.
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.
Some of the old monsters from Greek mythology that have a vendetta against Aphrodite's lineage could be the main antagonists. Maybe a spurned former lover of Aphrodite who was turned into a monster. This monster targets the daughter of Aphrodite and Percy because they are symbols of Aphrodite's power and influence. It constantly tries to harm them and thwart their plans.
The gods could potentially be antagonists. If the pregnancy goes against some divine law or prophecy, they might try to interfere. For instance, Zeus could see it as a threat to the order of things and send his minions to cause trouble for Percy and Bianca.
In a 'Percy and Aphrodite pregnant fanfiction', it could start with Aphrodite having a premonition about a special child that could only be born if she and Percy were to have a union. She then manipulates the situation in a very Aphrodite - like way to make it happen. Percy, on the other hand, is a hero with his own set of values. When he discovers Aphrodite is pregnant, he has to balance his loyalty to his friends, his place in the demigod world, and his new - found role as a potential father. The story could explore the power dynamics between a demigod and a goddess, as well as the themes of love, fate, and responsibility.
The other gods who oppose the relationship between Percy and the goddess could be antagonists. They might see it as a disruption of the natural order or a threat to their power. For example, if the goddess is from a certain pantheon and her pregnancy by a mortal like Percy goes against their rules, those traditionalist gods would be against it.
Another possibility is Hephaestus. Since he was Aphrodite's husband in the original mythology. In the fanfiction, their relationship could have taken an unexpected turn leading to the pregnancy. This could also add a layer of drama as Hephaestus might be overjoyed while Aphrodite could have mixed feelings due to her complex nature.
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.
It could be the navy. They see pirates as outlaws and would be determined to capture Percy and his crew. They might have better - equipped ships and more trained soldiers, making them a formidable foe for Percy and his fellow pirates.
Monsters could also be antagonists. With Percy in a new, more powerful state as a godling, there could be new and more powerful monsters that are drawn to his power. These monsters might be stronger and more intelligent than the ones he faced as a demigod, and they could be after his godly essence or trying to use him for their own dark purposes.