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.
Perhaps Naruto goes back and is able to change the way the Hidden Leaf Village treats the jinchuriki. This would lead to a more harmonious relationship between the village and those who carry the tailed beasts. It could also mean that Naruto doesn't have to suffer the same isolation and discrimination he faced in the original story.
One popular plot twist could be Killua going back in time and accidentally changing something that has a huge impact on his relationships. For example, he might prevent himself from meeting Gon in the first place, and then has to figure out how to get their friendship back on track. Another could be that he goes back to a time when a great evil was just starting, and he has to stop it before it gets out of hand, which completely changes the future he knew.
One possible plot twist could be Anakin going back in time and preventing his own fall to the dark side. He might use his knowledge of the future to avoid the events that led him to become Darth Vader. For example, he could avoid the temptation of Palpatine's offers. Another twist could be that his time - traveling disrupts the Force in unexpected ways, causing new alliances and enmities to form. Maybe he accidentally changes the course of other Jedi's lives, leading to a completely different galactic power structure.
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.
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.
Another plot twist could be a Sharingan user losing their eyesight but then gaining Byakugan - like abilities in a new way. Maybe through some experimental jutsu or a strange encounter. This would force them to adapt to a whole new way of seeing and fighting, and it would also create interesting interactions with other characters who are used to their old abilities.
One popular plot twist could be a character who is seemingly under the influence of Kyouka Suigetsu but is actually faking it to deceive the enemy. This adds an element of mystery and deception to the story.
One possible plot twist could be that Katniss goes back in time and prevents the Hunger Games from ever starting. Maybe she meets the people who first came up with the idea and manages to change their minds. Another could be that she goes back to a time before her father died and is able to save him, which would completely change her family's situation and her own personality development. A third plot twist might be that she goes back to a crucial moment in the Games and alters the outcome in a way that surprises everyone.
Perhaps the writer goes back in time thinking they can use their knowledge of the future to their advantage, but instead they find that the past has its own rules and forces that prevent them from doing so. For instance, every time they try to introduce a future concept, it gets distorted or erased by some mysterious force, adding a sense of mystery and frustration to the story.
Another twist might be Naruto going back to the time when the Nine - Tails attacked Konoha. He could potentially find a way to stop the attack or at least lessen the destruction. Maybe he could even change how the village views the Nine - Tails and those who are jinchuriki. For example, instead of fearing and ostracizing them, they could start to understand and accept them earlier.
One popular plot twist could be a shared enemy that forces the Naruto and Bleach characters in the harem to work together more closely. For example, a powerful new villain that has the ability to nullify both ninjutsu and reiatsu, making the characters rely on their combined wits and skills. Another might be a love triangle between a Naruto character, a Bleach character, and a new character created specifically for the fanfic. This adds drama and tension to the harem dynamic.