There's a possibility that Zuko would meet his grandfather. His grandfather's actions and decisions might have influenced the path the Fire Nation took towards war. By meeting him, Zuko could try to sway his grandfather's views and change the future of the Fire Nation at its root.
Well, one possible plot could be that Zuko goes back in time and tries to prevent the Fire Nation's aggression from the start. He could use his knowledge of the future to change things for the better, like trying to influence his father in a different way to avoid the war.
She could meet her younger self. This would be an interesting encounter as she could offer advice and guidance to her younger version. Maybe she could help her younger self to be less afraid of her powers.
He could meet his past self. It would be really interesting to see how he interacts with his younger, more naive self. He might try to give advice to his past self, like how to better control his power or avoid certain dangerous situations.
She could also encounter characters who were once enemies but are now in a different situation due to the time travel. For instance, she might come across a young Death Eater who hasn't fully committed to the dark side yet. Ginny could try to change their path and prevent them from becoming evil in the future.
He could meet his past allies in a different context. For instance, he might encounter Kakyoin or Polnareff when they were just starting their journey. Maybe he could also run into Dio at a time when Dio was still plotting his rise to power, which would create a very tense situation.
Lucy may also come across ancient warriors. For example, she could meet a Spartan soldier and be amazed by his combat skills and strict discipline. They could have a brief encounter where the Spartan is curious about this strange girl from another time.
Qrow might meet the founders of the Huntsman academies. He could learn from them the original intentions and philosophies behind the creation of Huntsmen. He could also offer his own insights based on his experiences in the future. This interaction could lead to significant changes in the way the academies are run and the training of Huntsmen in the past, which would then have a ripple effect on the future.
She could meet her younger self. It would be an interesting encounter as she could give advice to her younger version. Also, she might meet the earlier versions of her friends like Jiraiya and Orochimaru when they were all young and training together. Another possibility is that she could meet Hashirama Senju, the first Hokage, and learn from him directly about the founding of Konoha and his ideals.
Zuko's complexity makes him so interesting in these fanfictions. He has a troubled past which has shaped him, but he is constantly evolving. His ability to show vulnerability in front of the reader is another great trait. It allows for a deeper emotional connection between him and the reader, as seen in stories where he opens up about his insecurities and fears.
He could meet Dr. Halsey when she was just starting her research on the Spartan program. Master Chief could give her some insights from the future, which might change the way the program is developed.