Perhaps a rival ninja from another village who wants to use the situation to start a war. They might spread false rumors about the pregnancy being a result of some evil plot, causing unrest among the villages and trying to turn them against each other.
Another possibility is a jealous character within the Raikage's own village. They might have feelings for the Raikage and see Kushina's pregnancy as an obstacle. So they try to create problems for Kushina during her pregnancy, like trying to frame her for something or causing her harm.
It could be the elders of Kushina's village. They might see the pregnancy as a betrayal of their village and try to stop it. They could be afraid of the influence the Raikage's child would have on their own power structure.
One possible plot could be that the Raikage and Kushina have a secret relationship. Their pregnancy could be a shock to the ninja world as it crosses village boundaries. It might lead to political tensions, with some seeing it as a way to unite the villages through their child, while others fear it as a threat to the balance of power.
It could be a rogue ninja from the Cloud Village who has a grudge against the Raikage. This ninja might have been exiled unjustly in the past and now wants revenge, and Naruto gets caught in the middle of their conflict.
It could be an evil shinobi who wants to disrupt the meeting between Naruto and Kushina. Maybe they know that Kushina has some special knowledge or power that she could pass on to Naruto, and they don't want that to happen.
It could be a power - hungry ninja from within the Hidden Cloud Village who sees Raikage's relationship with Hinata as a threat to his own plans for taking over the village. He might try to spread false rumors about Hinata being a spy to turn the villagers against her and the Raikage.
The main antagonist could be the Hidden Leaf Village itself. Since Naruto was abandoned by Kushina, the village that failed to take proper care of him could be seen as the enemy. The village elders or those in power who turned a blind eye to his suffering could be the ones he has to oppose.
It could be Glimmer's over - controlling family. They might not approve of Cato and the pregnancy, and try to separate them. They could be the main source of conflict, constantly trying to manipulate Glimmer into getting rid of the baby or leaving Cato.
In some cases, it could be a powerful magical being from Sabrina's world. This being might think that the pregnancy is a bad omen or against some magical law. So, they try to stop the pregnancy or make Sabrina and Harvey's lives difficult by creating all sorts of magical obstacles for them.
It could be someone who took advantage of Kushina's leaving. Maybe a rogue ninja who wants to use Naruto's vulnerability to their advantage. They could try to turn Naruto against the village by making him believe that the village drove Kushina away.
It could be a rogue ninja from an old clan who has a vendetta against both Konoha and Kumogakure. This ninja has been gathering power in secret and plans to destroy both villages to avenge his clan's past defeat at the hands of the combined forces of the two villages.
It could be other villages that see the alliance between the Raikage and Naruto as a threat. For example, some of the more power - hungry villages might fear that the combination of the Raikage's strength and Naruto's abilities would tip the balance of power in the ninja world. So, they would try to undermine their relationship or even attack them directly.