Perhaps a really cool plot twist is that the time travel was actually a trap set by an old enemy. When Kakashi and Naruto travel back, they find out that this enemy has been manipulating events from the past all along. They then have to not only fix the past but also figure out a way to stop this enemy from using the past to destroy their future.
Well, a possible plot twist is that when they travel back in time, Sakura loses her medical skills. This forces her to learn them all over again in a more difficult environment in the past, and in the process, she discovers a new way of using medical ninjutsu that she brings back to the future. As for Kakashi, his Sharingan might behave differently in the past due to some temporal anomaly. This could lead to him having to control a new and unexpected power while also dealing with the enemies from that era. Also, they might find out that their time travel was actually set up by an enemy who wants to use their presence in the past to change the future to their advantage.
One interesting plot twist could be that Naruto, with his knowledge of the future, tries to prevent certain tragedies but ends up causing new ones. For example, he might try to save a character too early and disrupt the natural flow of events, leading to unforeseen consequences.
One interesting plot idea could be Kakashi traveling back in time to before the Uchiha massacre. He could try to prevent it, which would completely change the future of the Naruto world. Maybe he'd have to face off against Danzo and his schemes early on.
Maybe they travel back to their early days at the ninja academy. Naruto, who now has all his future knowledge, tries to befriend Sasuke in a different way. But Sasuke, being suspicious, thinks Naruto has some ulterior motive. This leads to a whole new set of comical and at the same time tense interactions between them as they both have to deal with their past selves and the changes they want to make.
One interesting plot twist could be that the Sannin travel back to a time when Orochimaru was still a good - hearted ninja. They could try to prevent his fall to the dark side. This would change the entire future of the Naruto world as Orochimaru's actions had a huge impact on many events.
Well, a cool plot twist could be that Naruto and Mito time travel to a period right before the Nine - Tails was sealed in Mito. Naruto, with his knowledge of the future, tries to find an alternative way to deal with the Nine - Tails' power. This could completely change the history of the Uzumaki clan and the Hidden Leaf. Also, they could time travel to a time when the first Hokage was still alive and Naruto has to convince him of future threats without revealing too much. Or perhaps they travel to a time when a powerful enemy was about to attack the village, and they use their future skills to defeat the enemy in a way that shocks everyone in the past.
One interesting plot twist could be that Naruto travels back in time and discovers that the Hidden Leaf Village was founded on a much darker secret than he ever knew. This changes his perception of the village's history and his role in it.
One plot idea could be that fem Naruto and Kakashi travel back to the founding of Konoha. They could use their modern - day knowledge and skills to influence the early development of the village in positive ways, like introducing new training methods. Another idea is that they travel to a time when a great enemy first emerged and they work to stop the threat before it becomes too powerful.
Imagine a fanfiction where Sakura, instead of being in love with Sasuke, has feelings for Naruto all along. This would completely change the love triangle aspect and lead to a lot of interesting interactions between the three main characters. It could also cause Sasuke to re - evaluate his relationship with his teammates.
One interesting plot twist could be that fem Naruto, instead of being the underdog like in the original story, is immediately recognized for her potential in the past due to her different appearance. Another could be that Sasuke, upon time traveling, tries to change things for the better but accidentally sets off a chain of events that makes the situation even worse. For example, he might try to stop certain tragedies from happening, but in doing so, creates new enemies or problems.