One plot idea could be Naruino traveling back in time to prevent a great disaster in their world. Maybe a powerful enemy once defeated them in the past, and by going back, they can change the outcome. Another could be traveling to the future to see how their actions in the present have affected the world. For example, they might find that their current choices led to a utopia or a dystopia.
Perhaps Naruino time travels to a point in the past where a great magical artifact was lost. Their mission becomes to find this artifact and bring it back to the present. Along the way, they meet ancestors or predecessors who teach them new magic skills that they can use to better protect their world in the present.
A cool plot could be that Naruino time travels and accidentally gets stuck in an alternate timeline. In this timeline, their friends have different personalities or roles. They then have to figure out how to get back to their original timeline while also dealing with the new versions of their friends. It could lead to some really interesting character development and interactions.
Maybe she travels to a time during a great war. She could end up changing the course of the war by providing crucial information or by using her skills in a way that no one expected. For instance, if she found a way to counter an enemy's powerful jutsu that was thought to be unbeatable at that time. This would not only affect the outcome of the war but also change the future of the ninja world she came from.
It could be a time - traveling villain from the future who wants to change the past to gain more power in their own time. They might try to stop Naruino from achieving their time - travel goals.
One idea could be Madara traveling back to the era of the Sage of Six Paths. He could try to change the very foundation of ninjutsu, which would create a whole new timeline. Maybe he would attempt to gain the power of the Sage for himself much earlier in history.
Well, perhaps a time - traveling device accidentally sends Judy Hopps back to a time when prey and predator relations were extremely tense. She has to use her wits and modern - day knowledge to prevent a major conflict that could have changed Zootopia's future for the worse. Or, a time - traveling fanfiction could focus on Nick Wilde going back to his own past, where he can make different choices that would impact his relationship with Judy and his standing in Zootopia society in the present.
One idea could be a character from the present in BO2 traveling back to a crucial moment in the game's history to change the outcome of a major battle. This would create a whole new timeline and a lot of interesting interactions with past characters.
Perhaps they travel forward in time instead. They could see the consequences of their actions in the future. Maybe they find out that their actions had unintended negative effects on some of their friends or the city as a whole. This would lead them to go back and try to fix things in a different way.
One idea could be Sauron travels back to the time before the forging of the Rings of Power. He could try to prevent the elves from ever having the knowledge to create such powerful items, thus changing the course of Middle - earth's history in his favor. Maybe he would target Celebrimbor directly, the elf - smith who was crucial in the creation of the rings.
In some narusasu time travel fanfiction, Naruto might go back to the time when Orochimaru was first targeting Sasuke. He could then use his skills and knowledge to keep Sasuke safe and away from Orochimaru's influence. A different plot could be Sasuke traveling to a time when Naruto was ostracized as a child and being the one to support Naruto, which would change their relationship dynamic from the start. Also, they could travel to a time during a major battle and use their combined future knowledge to turn the tides in their favor.
Another plot could involve Merlin traveling to the future. He might see a desolate Camelot, destroyed by some unknown force. Then he has to return to his own time and figure out how to prevent this dark future from happening. This would involve him using his magic in new and creative ways to influence the present to change the future.
One idea could be Sasori traveling back to his childhood and trying to change his path towards becoming a puppet. He might attempt to save his parents and avoid the tragedies that led him to his cold-hearted ways. Another plot could be Sasori traveling to a future where the Akatsuki has achieved its goals, only to find that the world is in a state he didn't expect, so he has to go back and change the Akatsuki's plans. Or he could travel to different ninja villages' pasts to steal their most powerful puppet - making secrets.