Elsa might meet the previous rulers of Arendelle. She could learn from their leadership and wisdom, and perhaps apply it to her own rule when she returns to her own time. This could make her an even better queen.
There's a possibility that Elsa meets characters from the kingdom's past that she never knew before. For example, she could come across an old wizard or sorceress who has knowledge about powers similar to hers. This encounter could open up new ways for Elsa to develop and control her powers, and also add more mystery and depth to the story of Arendelle's history.
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.
Perhaps Elsa goes back in time to meet her parents again. She could learn more about her powers from them, as they might have known more than she initially thought. This encounter could also give her a chance to say goodbye properly, which she didn't get the first time. And it could lead to her having a better understanding of her place in the kingdom and her family.
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.
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.
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.
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.