She could meet the founders of Hogwarts. Imagine Hermione having a conversation with Rowena Ravenclaw about the importance of knowledge and wisdom. She might also meet Godric Gryffindor and learn about the true meaning of courage from him. Another person she could meet is Salazar Slytherin. It would be very interesting to see how she would interact with him, given his views on blood purity.
She may encounter the ancient magical creatures that are now rare or extinct. For example, she could meet a real phoenix in its prime, or come across a group of centaurs from a time when they were more involved in the wizarding world's affairs. Hermione might also meet some of the early potion - makers and learn from them, like the ones who first discovered the properties of the Philosopher's Stone. This would expand her knowledge of potions even further.
In the past, Sakura could meet the original creators of the magic system she uses. They might be able to teach her things that have been lost over time. She may also encounter a young, talented artist or inventor who was never mentioned in the history books. This person could be creating something that will have a big impact in the future. And there's always the possibility that she meets a kind - hearted stranger who becomes her best friend in the past and has a lasting impact on her life, even when she returns to her own time.
In this fanfiction scenario, Goku could meet Bulma when she was a child. He could befriend her in a different way compared to their first meeting in the original story. Also, he could run into King Kai at an earlier time and start training with him much sooner, which would significantly boost his power levels and change the course of his battles in the future.
She could meet her future self. This would be an interesting encounter as they could compare their experiences and knowledge. Her future self might give her some advice on things she should do or avoid in her own time. Another person she could meet is a descendant of her friends. For example, a grandchild of Harry and Ginny who has inherited their bravery and magical skills.
Korra could meet the founders of the different bending disciplines. She could meet the first water benders and learn about the purest form of water bending. This would not only enhance her own skills but also give her a deeper connection to her water - bending heritage.
She could go back in time to the Triwizard Tournament and use her knowledge of future events to prevent Cedric's death. Maybe she figures out a way to expose Barty Crouch Jr. much earlier. Another plot idea is that Hermione travels back to the time when the Marauders were at Hogwarts. She could interact with them, and perhaps try to influence Sirius Black's fate, making sure he doesn't end up in Azkaban. She might also try to learn more about Remus Lupin's lycanthropy from a different perspective.
In some cases, the antagonist could be Esme herself, or rather a version of her from the past. The past Esme might be more naive or self - centered and not understand the implications of Esme's actions in the past. The present Esme is trying to make positive changes, but the past version of herself keeps getting in the way, either through her own actions or by influencing the people around her in a negative way.
It could be a time - traveling version of Zeref. He might be trying to stop Natsu from changing things in the past that could potentially lead to his own downfall in the future.
It could be a time - traveling pirate who has his own evil plans for the past. He might be trying to change the past to gain ultimate power in the future. So he becomes an obstacle for Luffy who wants to either preserve the past or make positive changes.
Danzo could still be a major antagonist. Even if Naruto travels back in time, Danzo's thirst for power and his underhanded methods would still pose a threat. Naruto would have to find a way to deal with him without causing too much chaos in the village.
It could still be Orochimaru. Even if Sasuke travels back in time, Orochimaru's thirst for power and knowledge wouldn't change. He would still be plotting to gain power, and Sasuke, with his knowledge of Orochimaru's future actions, would be in a position to stop him. Orochimaru's experiments and his desire to take over the bodies of others make him a formidable antagonist.