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.
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.
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.
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 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.
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.
A new character could be introduced as the antagonist. Maybe someone who has also time - traveled and has their own agenda for the past. This character could be trying to preserve the original timeline for some reason, or they could be trying to create an even worse future. They would oppose Obito and Kakashi at every turn, creating a lot of conflict and drama in the fanfiction.
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.
It could be Orochimaru from an earlier time. Since he was always after power and secrets, Naruto's knowledge of the Rasen Shuriken and his time - traveling abilities would be a huge threat to Orochimaru's plans. Orochimaru might try to steal Naruto's jutsu or use him to gain more power.
Danzo could be a great antagonist. If Naruto travels back in time, Danzo's schemes and thirst for power would still be there. He might see Naruto as a threat to his plans of controlling Konoha and try to eliminate him. His underhanded methods and shadowy operations would make him a formidable foe for Naruto, especially when Naruto is trying to change the past for the better.