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.
Goku might run into past versions of his friends. He could see a younger Krillin or Bulma and have some really interesting interactions. He could also meet past versions of his enemies and try to change their fates. For instance, he could meet a young Vegeta and try to steer him away from the path of evil. This would be a huge change in the Saiyan's story.
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.
She might meet her younger self. This could be a really strange encounter as she tries to understand how she used to be.
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.
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.
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.
In the 'Sora Goes Back in Time' fanfiction, Sora may run into his friends at different points in their lives. He could meet Riku when they were just kids, and see how their friendship started in a whole new light. He might also come across Kairi at a crucial moment in her past, and his presence could change the way she develops as a character. There could also be encounters with other minor characters from the original story, whose roles could be expanded upon in this time - traveling scenario.
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.