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.
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.
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.
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.
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.
Danzo could be a main antagonist. He's always been power - hungry and has seen Naruto as a threat to his control over the village. After Pain's attack, he seizes the opportunity to turn the villagers against Naruto.
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.
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.
The village elders could be the main antagonists. They might see Naruto's godlike powers as a threat to their traditional way of ruling the village and decide to get rid of him. Another option could be a secret organization within the village that wants to use Naruto's power for their own evil purposes and when he refuses, they turn on him.
Naruto may turn more cautious. The Sasuke retrieval was a dangerous and complex situation. In fanfiction, it could make him think twice before jumping into action. He may analyze situations more carefully to avoid potential disasters. For instance, he won't be as quick to engage in a fight without considering all the possible outcomes. This is a big change from his usually head - first approach. He may also become more protective. Having almost lost Sasuke, he could be overprotective of his friends and the village, constantly on guard to prevent any harm from coming to them.
Obviously, the gods themselves would be the main antagonists. They have the power to manipulate Naruto's world and his fate. They could be some sort of ancient deities who have been watching over the ninja world from afar and now feel the need to interfere.