Well, perhaps Darth Vader goes back in time to the very beginning of the Sith. He then discovers secrets about the origin of the dark side that he didn't know before. This new knowledge could make him question his loyalty to Palpatine. He might decide to try and change the Sith's path from the start, which would be a huge plot twist. He could also find that his own existence in the past has unforeseen consequences, like creating a rift in the Force that starts to affect the balance in ways no one could have predicted.
A great plot twist could be that Darth Vader travels back to the time when the Jedi Order was at its peak. His dark side presence is detected by the Jedi Council immediately. However, instead of being able to simply capture or kill him, they find that his knowledge of future events gives him an edge. He might use this to manipulate the Jedi, creating internal strife within the Order. This could also lead to some Jedi being tempted by the dark side because of the power that Vader represents. And all of this could ultimately reshape the future of the galaxy far more than anyone expected.
A plot twist could be that Darth Vader actually reveals his true identity to Luke and Leia much earlier than in the original story. He tells them that he is their father not as a last - ditch attempt to turn Luke, but as a way to protect them from the Emperor. He then uses his knowledge of the dark side to train them in how to defend against Sith attacks. This changes the whole dynamic of their relationship and they end up forming an unlikely alliance with Vader against the greater evil of the Emperor.
In such a fanfiction, Darth Vader might try to prevent his fall to the dark side. He could use his knowledge of the future to avoid the events that led him to become a Sith. For example, he might prevent the death of his mother or his duel with Obi - Wan on Mustafar.
Well, one possible plot could be that Darth Vader tries to persuade Luke to join the dark side once again. Maybe he shows Luke some 'truths' about the Jedi that he believes are lies, trying to shatter Luke's faith in the Jedi way.
One possible plot twist could be that Naruto, when he travels back to graduation, realizes he has some of his future skills and knowledge. He might use this to outperform everyone in the exams in unexpected ways, like using advanced jutsu that no one has seen before, which would shock his classmates and teachers.
Well, in a 'Darth Vader goes back to the Clone Wars' fanfiction, he might seek to alter the relationships within the Jedi Order. He could try to influence Anakin Skywalker (his former self) in a different way. For example, he could try to expose the Sith plot earlier, which would completely change the dynamics of the Clone Wars. He might also try to use his advanced combat skills and knowledge of the dark side to turn the tide of certain battles in a different direction than what originally happened.
A possible plot twist could be that when they travel back, they find out that the past is not as they remembered it. Maybe some events were actually caused by an unknown enemy manipulating things from the shadows all along.
He could meet his younger self, Anakin Skywalker. That would be a really interesting encounter. He might also meet Qui - Gon Jinn, who could sense the conflict within him. And there's a chance he'd cross paths with Yoda in his prime, which would lead to some intense confrontations or perhaps even unexpected alliances.
Another plot twist could be related to Kakashi's own abilities. Since he has knowledge of the future, he might try to use some jutsu or skills that he hasn't learned yet in his original time. But this could come with a cost, like draining his chakra too much or attracting unwanted attention from powerful enemies. He could also accidentally teach his past self some skills that change the course of his own development.
Well, perhaps a plot twist could be that when Kirito goes back in time, he discovers that his own existence in the past was a key factor in creating the problems he faced in the future. So, he has to figure out a way to change things without erasing himself. Or, he could go back and find that the person he loves in his original timeline has a completely different personality or role in the past, which throws him off and makes his feelings really complicated.
In such a fanfiction, Darth Vader could change the course of history. He might prevent the rise of the Empire or alter his own fall to the dark side. For example, he could warn Anakin Skywalker about Palpatine's true intentions, thus changing the entire Star Wars timeline.