There could be a new character introduced as the antagonist. A powerful figure who has a vendetta against the Uzumaki family. They might have been the reason Kushina left in the first place, either by directly forcing her or by creating a situation that made her feel she had no choice. And now they set their sights on Naruto.
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 an old enemy of Kushina's who was thought to be dead but has now returned. Maybe someone from her days in the Uzumaki clan who has a vendetta against her and now sees Naruto as an obstacle too.
It could be Orochimaru. He's always been interested in powerful ninja and their abilities. Since Kushina has strong chakra and a connection to Naruto, Orochimaru might see her as a valuable asset to either experiment on or use as bait for Naruto. He's devious enough to come up with a complex plan to capture her.
It could be Orochimaru. He might have manipulated Kushina into kidnapping Naruto. Orochimaru has always been interested in the power of the Uzumaki clan and Naruto's nine - tailed fox power. He could be using Kushina as a tool to get to Naruto.
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.
Maybe an evil organization that wants to use Naruto's power and the connection he has with Kushina for their own dark purposes. They see Natsumi as an obstacle in their plan because she is always by Naruto's side. This organization could be experimenting with dark jutsu and need Naruto's chakra and Kushina's latent abilities (passed on to Naruto) to complete their evil experiments.
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 a powerful ninja from another village who sees Naruto's godlike powers as a threat to his own plans of domination. Since Naruto is already feeling vulnerable due to his family leaving him, this antagonist might try to take advantage of his emotional state.
It could be a rogue ninja from a hidden village that has a grudge against Naruto. This ninja might have a special jutsu that specifically targets Naruto's left eye during a fight, causing him to lose it.
It could be that there is no traditional antagonist. The conflict might be internal, with Naruto's own emotions towards Kushina being the main source of drama.
It could be a jealous fellow trainee. This person might spread false rumors about Naruto, which leads to him being left behind in training. The jealous trainee doesn't want Naruto to get stronger and be more popular.