Well, it could be that the other dimension has a time - warp. Naruto enters and finds that he is in the past or future of his own world. If it's the past, he might accidentally change things that could have a huge impact on his own dimension. In this new dimension, there could also be a different form of the tailed beasts. Maybe they are combined into one super - powerful beast. Naruto's encounter with this new form could be a major plot twist as he tries to figure out how to deal with it, either befriend it or fight it.
One possible plot twist could be that in the other dimension, Naruto finds out that all his friends have different personalities. For example, Sasuke could be a peaceful scholar instead of a brooding avenger. Another twist might be that the chakra system works completely differently in this new dimension. Maybe instead of using hand signs, they use some kind of mental power to perform jutsus. Also, Naruto could discover that the Akatsuki in this dimension are actually the good guys, fighting against an evil ninja clan that wants to take over the world.
Well, perhaps Natsu could find himself in a dimension where magic works completely differently. He might have to learn new ways to use his fire magic or even discover new types of magic altogether. Maybe he meets alternate versions of his friends who have different personalities due to the different environment of that dimension.
His relationship with Sasuke would be very different. In the new dimension, Sasuke might not have the same vendetta against his brother. So Naruto wouldn't have to worry about Sasuke going off the rails. With Sakura, she could be a powerful ninja in a different way, like she might be more focused on medical ninjutsu and Naruto would see her in a new light. And his relationship with Kakashi could change too. Maybe Kakashi is a strict military - like leader in the other dimension instead of a laid - back teacher.
Naruto could meet alternate versions of his friends or enemies. For example, he might find a version of Sasuke who has a completely different personality or a different set of skills. This would create interesting interactions and potential conflicts or alliances.
Well, perhaps Percy could find himself in a dimension where the laws of physics are completely different. He might have to learn new ways to use his powers. For example, if water behaves strangely there, like it's solid in some areas and gas in others, he'd have to adapt quickly.
One possible plot twist could be that the new Naruto has different abilities. Maybe he has a unique form of chakra control that the original Naruto didn't have. This could lead to different battle strategies and interactions with other characters. For example, in a fight against Sasuke, he might use his new chakra control to create unexpected jutsus.
A group of rogue ninja from the other dimension could also serve as the main antagonists. They might be jealous of Naruto's power from his original dimension and want to steal it for themselves. They could be led by a charismatic and devious leader who has a personal vendetta against Naruto for some reason, like he believes Naruto will disrupt their way of life in the dimension.
One idea could be Naruto being transported to a dimension where chakra doesn't exist, but he discovers a new form of energy and has to learn to use it from scratch. This would force him to adapt his fighting style completely.
Well, a really interesting plot twist in an Eren travels back in time fanfiction could be related to relationships. Say Eren goes back and realizes that his actions in the past have a huge impact on his relationships with his friends. Maybe he accidentally causes Mikasa to not be as attached to him as she was in the original timeline. Or he could find out that Armin was supposed to be the one with the time - traveling ability all along, and his interference messes up Armin's future plans. Also, what if he goes back and discovers that the real enemy wasn't the Titans but some other force that was using the Titans as a distraction, and his time - traveling journey is the only way to expose this truth.
A plot twist could be that Sakura travels back in time and realizes that the world she thought she knew was all a lie. The history books in her time were wrong. She could find that the origin of her magical powers was something very different from what she was taught. Also, she might go back to a time when there was a powerful artifact that was lost in her own time. However, when she tries to retrieve it, she discovers that it has a dark side that she didn't expect. This could lead to a moral dilemma for her, whether to keep it or leave it be.
He might meet a powerful wizard who is the ruler of that dimension. This wizard could initially be hostile towards Natsu, seeing him as an intruder, but later they could become allies.