Some of Naruto's former friends who have been influenced by the negative propaganda against him could also be antagonists. They might be misled into believing that Naruto has changed for the worse. They could actively oppose Naruto, either by trying to capture him or by preventing him from doing certain things. Another possibility is that there could be a hidden group within the village that has a vendetta against Sasuke and, by extension, Naruto for bringing him back.
It could be the ninja council as well. They are in charge of making important decisions for the village. If they perceive Naruto as a potential danger after the Sasuke retrieval, they could order his arrest or exile. They might be worried about the power dynamics in the village changing because of Naruto's new - found status or the influence of Sasuke. And they would do whatever it takes to keep the village 'safe' in their view, even if it means betraying Naruto.
Well, perhaps in the fanfiction, after bringing Sasuke back, Naruto discovers that there are some secrets that the village has been hiding from him all along. When he tries to expose these secrets, the village sees it as a betrayal and turns against him. He might then be forced to team up with some unexpected allies outside of the village to clear his name and fight for what he believes is right. It could also be that Sasuke's return actually brings some kind of curse or danger with it, and Naruto is wrongly blamed for it.
The relationship between Naruto and Sasuke is complex. In this retrieval fanfiction, their past memories together could play a big role. Flashbacks of their time at the ninja academy, their first missions, and the bond they formed could be used to heighten the drama of Naruto's quest to bring Sasuke back.
There could be an outside force, like a neighboring village that wants to keep Naruto and Sasuke away from their own village. They might be afraid of the potential power and influence Naruto and Sasuke could bring back if they return. So, this neighboring village sends their best ninjas to hunt down Naruto and Sasuke during their banishment, making them the main antagonists in the fanfiction.
It could be some of the village elders. They were the ones who decided to banish Naruto, so they could continue to oppose him even when he's in exile. They might see his growing power and influence as a threat to their traditional ways of running the village.
Friendship is a very common theme. Naruto's strong bond with Sasuke is at the heart of the retrieval arc in the original story, and fanfictions often explore this further. They show how Naruto's love for his friend drives him to go to great lengths to bring Sasuke back.
In some fanfictions, Naruto may become more mature. After going through the arduous process of retrieving Sasuke, he realizes the complexity of relationships and the importance of his comrades more deeply. He might also become more reserved in his emotions, as the experience has made him more aware of the consequences of his actions and words.
It could be an original character created just for the fanfiction. Maybe a powerful ninja from a forgotten clan who has a vendetta against Konoha and targets Naruto, Sasuke and Sakura specifically.
In some fanfictions, a new character could be created as the main antagonist. This character might be jealous of Naruto and female Sasuke's growing relationship and power. They try to create rifts between them and stop them from achieving their goals.
In this kind of fanfiction, the Fallen Angels could be main antagonists. They might see Naruto and Sasuke as threats to their plans in the DxD world.
Well, Naruto would probably face a lot of tough opponents. Since Sasuke has his own goals and might be with some powerful people, Naruto would have to fight his way through. He might encounter new jutsus and strategies that he has to overcome.