Maybe an old enemy of Naruto resurfaces. They see Sarada's pregnancy as an opportunity to get revenge on Naruto. They think that by harming Sarada or her unborn child, they can finally break Naruto's spirit. However, they underestimate the bond between the village and Sarada, and Naruto's determination to protect her.
One potential antagonist could be a group of rogue ninjas who see Sarada's pregnancy as a sign of weakness in the village. They might try to target her to cause chaos. Another could be a jealous rival who doesn't like the attention Sarada is getting due to her pregnancy and her connection to Naruto.
Perhaps there are some anti - Uchiha elements within the village that are not happy with Naruto adopting Sarada. They could try to undermine her training or create problems for Naruto and Sarada. They might spread false rumors or try to isolate Sarada from the rest of the village.
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.
One potential antagonist could be those within the village who oppose Naruto's relationships. They might see his harem as a threat to the traditional values of the village and try to cause trouble for Naruto and his partners.
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.
The Brotherhood of Mutants could also be antagonists. They might view the pregnancy as a sign of the X - Men becoming too domesticated and vulnerable. They could launch attacks on the X - Mansion in an attempt to disrupt Rogue and Logan's new - found family situation.
Maybe there are some traditionalists in their world who don't approve of their relationship or the pregnancy. These people could be the antagonists, constantly trying to break them apart or cause problems for them.
Frieza or his remnants could be antagonists. They might see the unborn child as a potential threat in the future, especially if it has strong Saiyan powers. So they could plan to attack Bulma to prevent the birth.
The Vicious 6 could be the antagonists. They might see Gru's preoccupation with Lucy's pregnancy as a weakness and try to take advantage of it. They could plan to kidnap Lucy to get to Gru and force him to give up his new - found family life and return to his old villainous ways.
Some of the power - hungry Underlanders who oppose Luxa's rule could be antagonists. They might try to use her pregnancy as a way to undermine her authority. They could spread rumors or create unrest, believing that with Luxa being pregnant, she is in a weaker position to lead, and they could try to take over the throne.
One possible antagonist could be a jealous sorcerer. He might be envious of Hiccup and Elsa's relationship and the power that their unborn child is said to possess. So he tries to harm Elsa during her pregnancy to stop the child from being born. Another could be some of the traditionalists from their respective kingdoms. They don't approve of their relationship and see the pregnancy as a threat to their old ways. They might try to separate Hiccup and Elsa, which would put Elsa's pregnancy at risk.