Shang Tsung could be a great antagonist. He might see Sindel's pregnancy as an opportunity to gain power. Maybe he wants to use the unborn child for some dark magic or experiment. His devious nature would make him a formidable foe against a pregnant Sindel.
Quan Chi could be another potential antagonist. He is always looking for ways to further his own dark agenda. The idea of a powerful being being born from Sindel might not sit well with him. He could scheme to either corrupt the unborn child or get rid of Sindel and the baby before they become a problem for his plans.
Outworld's emperor Shao Kahn could also be an antagonist. He might view Sindel's pregnancy as a threat to his rule. If the child has the potential to be a powerful force, he would want to eliminate the threat. He could try to manipulate Sindel or directly attack her to prevent the birth of the child.
A possible plot is that Sindel's pregnancy gives her new powers or a different perspective on life. She could start to see the Mortal Kombat battles in a new light, perhaps trying to find more peaceful solutions to conflicts as she doesn't want her child to grow up in a world full of constant fighting. This could put her at odds with some of the more war - hungry characters in the Mortal Kombat world.
Ra's al Ghul, if he's still alive in the fanfiction, could be an antagonist. He might have different plans for Talia's future and the baby that don't align with what Talia wants. Maybe he wants the child to be raised in a certain way that Talia disagrees with, leading to a conflict between father and daughter.
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.
Victoria Grayson would be a prime antagonist. In the 'Revenge' series, she was always at odds with Emily. If Emily is pregnant, Victoria might see this as a weakness and try to use it against her. She could spread rumors or try to harm Emily in some way to stop her revenge plot.
A former acquaintance of Beckett who has turned to a life of crime could be an antagonist. They might have some old grudge against Beckett, like Beckett once reported them for something minor but it led to them having a really hard time, so now they want revenge by holding Beckett hostage.
Another antagonist could be a group within the ninja village itself. Maybe they fear the changes that the child of prophecy might bring. They could be the more conservative elements who don't want to see new ideas or powers emerging. They might try to undermine the child's training or spread false rumors about them. For example, they could claim that the child is a danger to the village, turning public opinion against the child.
The established nobles who fear losing their power could be antagonists. They might try to plot against the new king to keep their old ways and privileges. Another potential antagonist could be a powerful and jealous rival who believes he should be the king instead.
Liu Kang is also a possibility. In some fanfiction scenarios, the gender roles can be a bit more fluid. His noble and powerful presence could be part of the harem. Then there's Scorpion. His loyalty and intensity could make him an interesting addition to the harem around the OC. There could be a lot of drama with his vendetta-filled past.
It could be some of the hospital's board members who might not approve of their relationship and Cameron's pregnancy while working at the hospital. They could try to make things difficult for the couple.
In Erza's world, there could be a dark mage who wants to use the power of the unborn child for their own evil purposes. They might see the combination of Naruto's ninja abilities and Erza's magic in the child as a great source of power and will stop at nothing to get their hands on it.