It could be a time - traveling rogue ninja from the future who wants to change the past to gain power. This ninja might try to stop Hinata's mom from achieving her goals in the past.
Well, it could be about them traveling back to prevent a great disaster that was supposed to happen in the future. Maybe something related to the destruction of the Hidden Leaf.
One great one I found has Naruto and Hinata's mom team up to change certain events in the past. It's really interesting to see how their different perspectives and knowledge of the future play out.
It could be a future version of himself. If Dark Hinata travels back in time, his future self might see this as a threat to the timeline he has created and try to stop him. This would create an interesting internal conflict within the character.
One idea could be that Hinata's mom travels back in time and tries to prevent certain events that led to hardships in Hinata's life. For example, she could stop the discrimination Hinata faced in the Hyuga clan early on.
It could be a future version of Orochimaru who has found a way to come back even more powerful. Maybe he has developed some new form of immortality and is causing chaos in the future world that Naruto travels to.
It could be Orochimaru. Imagine Orochimaru discovers baby Naruto's time - traveling ability and wants to use it for his own evil purposes. He might try to manipulate baby Naruto to change the timeline in a way that benefits him, like making it easier for him to gain more power or revive his dead comrades.
It could be Orochimaru. If Naruto and Kiba travel back in time, Orochimaru might be in the middle of his evil experiments. He was always a major threat in the Naruto series, and in the past, he could be even more dangerous as he was less known and had more freedom to act. He might target Naruto and Kiba to use them in his experiments or to stop them from interfering with his plans.
Madara Uchiha could also be a main antagonist. If Naruto goes back to a time when Madara was still alive and plotting, Madara would surely oppose Naruto's attempts to change the future. He has his own grand plans for the world and wouldn't let Naruto interfere easily.
In many such fanfictions, Naruto and Hinata might be sent back in time through some magical or jutsu - related means. Once they meet their parents, it could be a very emotional moment. Naruto might see his father Minato and mother Kushina, and Hinata could meet her father Hiashi. They could learn a lot about their families' pasts and perhaps even change some things for the better in the timeline.
Danzo could be a great antagonist. If Kakashi travels back in time, Danzo's underhanded plans, like his control over Root and his desire for power, would still be in motion. He'd likely oppose Kakashi's attempts to change things for the better, especially if it threatened his own influence.