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.
Master Chief might meet the Forerunners. Since they are an ancient and powerful race in the Halo universe, this encounter could be very significant. He could learn from them or even influence their actions, which would have a huge impact on the future of the galaxy.
He could meet some of the early Covenant leaders. By interacting with them, he could try to prevent the formation of the Covenant or at least change its aggressive nature. Maybe he could convince them to co - exist with humanity peacefully, which would be a complete change from the original storyline.
One possible plot could be Master Chief going back to a time before the Covenant war. He could use his knowledge and skills to prevent certain events that led to the war. For example, he might stop the discovery of the Halo rings by the Covenant in the first place, which would change the entire future of the galaxy.
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 Peggy Carter. Since he had a strong connection with her in the past, it would be a very emotional reunion. They might share some unspoken feelings that were left unexpressed before.
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.
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.
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.