One main antagonist could be a rogue ninja from a hidden village. This ninja has a vendetta against Konoha and targets Naruto and Hinata specifically because of their importance to the village. He uses powerful dark jutsu to try and stop them in their adventures.
A group of international criminal ninjas could be the antagonists. They are after a powerful artifact that Naruto and Hinata are also tasked with protecting. These criminals have various unique and dangerous abilities, and they constantly try to outwit and overpower Naruto and Hinata throughout their adventure.
One possible main antagonist could be Madara Uchiha from an earlier time period. If Naruto and Kyshina travel back far enough, they might encounter a younger and more power - hungry Madara. He could be a major obstacle as he tries to achieve his early goals of creating a world under his control, which would be in direct opposition to what Naruto and Kyshina are trying to do to protect the future of the village.
There could be other ninja from rival villages who act as antagonists. They might see Naruto as a threat and target Hinata to get to him during the Chunin Exams. These antagonists would create conflict and danger for Naruto and Hinata, forcing them to grow and rely on each other more.
One possible antagonist could be a jealous student. Maybe someone who has a crush on Hinata and doesn't like Naruto getting close to her. This student could try to sabotage their projects or spread false rumors about them at the academy.
Another type of antagonist could be a strict professor who doesn't like Naruto's carefree attitude and Hinata's association with him. The professor might try to give them a hard time, like assigning them extra difficult projects or being overly critical in class, which becomes an obstacle in their college life and relationship.
It could be some rogue ninjas who are against Naruto's new peaceful policies as Hokage. They might see Hinata as a weakness to be exploited to get to Naruto.
The main antagonists could be other powerful clans in the Hidden Leaf Village. For example, the Uchiha clan (if they were still around and had some grudges or power struggles). They might see the restoration of the Hinata clan as a threat to their own status and power within the village.
Maybe Orochimaru's former followers who are still lurking in the shadows. They see Naruto as a threat and Hinata as an easy target to get to Naruto. Another possibility could be some power - hungry shinobi from other villages who view Konoha as weak after the Pain incident. They might target Naruto and Hinata as key figures in Konoha's future to try and undermine the village's rebuilding efforts.
It could be some new characters created specifically for the fanfic. Maybe a group of rogue ninjas who are after a powerful artifact that Naruto and Hinata are also trying to protect.
It could be some of the old - school ninja clans who are against the new order Naruto and Hinata are trying to establish. They might see their rule as a threat to their traditional power and influence.
Perhaps a rogue ninja group that has a vendetta against Anko. When she forms this new family with Naruto, they target the whole family unit to get back at her. They might see Naruto as a way to get to Anko or vice versa, and constantly attack and create chaos for them.