One idea could be Ulquiorra and Orihime getting sent back to the early days of Soul Society. They could end up changing the course of events that led to certain battles. For example, they might prevent the initial conflict between the Soul Reapers and the Hollows in a way that alters the power dynamics in the Bleach universe.
A possible plot is that they time - travel to a future where the Soul Society has been completely destroyed. They then have to figure out what led to this disaster and try to change the past to prevent it. Along the way, they might discover secrets about their own powers and the true nature of the Bleach world that they never knew before.
Another plot could involve them traveling to a time before Ulquiorra was an Arrancar. This would give Orihime a chance to see a different side of him, perhaps a more human side. They could then try to find a way to prevent his transformation into an Arrancar, which would have a huge impact on the future of Hueco Mundo and its relationship with the Soul Society.
In a time - travel situation, Orihime could have a chance to understand Ulquiorra better. She might see his past and the events that shaped him into the being he is in their original timeline. This new understanding could lead to a deeper emotional connection between them. She could start to see him not just as an enemy but as a complex being with a tragic past, which would change the way she interacts with him.
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.
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.