It could be Orochimaru. Since Orochimaru has always been interested in power and immortality, if Naruto goes back in time, Orochimaru might sense the change in the timeline and try to stop Naruto from changing things that could affect his future plans for power.
His relationship with Sasuke would be very different. If Naruto stops the Uchiha massacre, Sasuke may not have the intense hatred he had in the original story. They could grow up as friends without the need for Naruto to constantly chase after Sasuke to bring him back.
The plot could also involve the other characters' reactions to this evil version of Naruto from the future. Some might be confused, while others might immediately sense the danger. Naruto could also face challenges in changing the past as there could be unforeseen consequences or time - related ninja techniques that prevent him from fully achieving his evil goals.
One idea could be Naruto going back in time and preventing the Uchiha massacre. He could use his knowledge of future events to change things for the better in the Uchiha clan, and this would also have a huge impact on Sasuke's character development.
The main antagonists could be the original Naruto's friends and allies. Since this is an evil Naruto, people like Sasuke, Sakura, and Kakashi would likely oppose him and try to stop him from whatever evil plans he has in the past.
One popular plot twist could be Naruto preventing the Uchiha massacre. This would completely change the future as Sasuke's path of revenge would be derailed. It would also impact the relationship between the Uchiha and the rest of the village.
One possible reason could be to correct some mistakes in the future. Maybe there were some events that led to a really bad outcome, like the destruction of the Hidden Leaf Village, and Kami thought sending Naruto back in time could prevent it. Another reason might be to give Naruto a chance to learn more about his predecessors and gain new powers or knowledge that could help him in his original timeline.
Well, the story might progress as they try to blend in with their younger selves' lives. Naruto would still be the go - lucky guy, but with a more serious undertone because he knows what's coming. Hinata would be more confident perhaps. They could start to change things like the way ninjas are trained in the village. They might introduce new techniques they learned in the future.
In such a fanfic, Naruto and Hinata getting sent back in time could lead to some really interesting relationship developments. They could start their relationship much earlier, and since they already know each other well from the future, they might have a deeper connection right from the start. Also, they could use their knowledge of future events to gain allies more easily.
One common plot twist could be that the female Naruto discovers she has a hidden power that was dormant in her original timeline but is awakened in the past. For example, she might find out she has a special connection to an ancient jutsu or a long - lost clan. Another plot twist could be that her actions in the past change the future in unexpected ways. Maybe she accidentally prevents the birth of one of her important friends in the original timeline. Or perhaps she gets caught up in a political intrigue that was not part of her original story and has to navigate through it carefully to ensure she can still return to her own time and that the future she knows is not completely altered.