He might also run into his brother, Gregor Clegane, at a time before their animosity fully developed. This could lead to some really interesting scenarios. Sandor could try to change the course of their relationship, either by preventing the events that made Gregor so cruel or by finding a way to co - exist with him in a more peaceful way.
He would probably encounter himself at different ages. Meeting his younger self could be a very intense and strange experience. He could try to give advice to his younger self, but his younger self might not be receptive.
One possible plot twist could be Sandor traveling back to his childhood and preventing the incident that scarred him. This would completely change his character arc and his relationship with his brother. He might become a more confident and less bitter person.
Eleven could be a likely candidate. Given her powerful psychic abilities, she might accidentally or purposefully time travel. Her powers have always been a bit unpredictable, and time travel could be an extension of that. She has a strong connection to the Upside Down, so she might be sent back in time to learn more about its origin or how to stop it.
I think Fundy would be likely to time travel. He has a complex relationship with his family and the SMP. Time travel could be a means for him to understand his father better or change the way his family relationships developed. GeorgeNotFound could also be a candidate. He's often in the background but has a strong connection to Dream. Time travel might be his way of getting more involved in the SMP's events and helping Dream in a different way. Lastly, Niki could be a time traveler. She has been through a lot of changes on the SMP, and time travel could be her opportunity to go back and make the SMP a more inclusive and friendly place from the start, especially for those who have been marginalized.
I think Yamato could be a good choice too. His ability to control wood style could be really useful in the past. Maybe he could time - travel to help with the growth of the first trees in the ninja world or something like that. And his calm and collected nature would help him adapt to the new situation in the past.
Midoriya would be a likely candidate. He's always looking for ways to improve and become a better hero. Time travel could give him the chance to learn from past heroes directly, like All Might in his prime. He could also go back to understand more about One for All's past users and how to master the quirk faster.
Ban could be another one. He has his own backstory related to the sins and his immortality. He might time travel to find a way to break free from his immortality curse or to go back and correct a mistake he made when he was under the influence of one of the sins, like greed when he stole something important in the past.
Star - Lord is a likely candidate. He's always been a bit of a risk - taker and his curiosity might lead him to experiment with time travel technology.
I think Tyrion Lannister would be an interesting character to time travel in a fanfiction. He's very intelligent and cunning. Time travel could be his way to outwit his family and those who have wronged him. Arya Stark could also be a likely character. She has the skills of a faceless man. If she could time travel, she could use her skills to change the past, perhaps by assassinating key enemies before they cause harm. And of course, Melisandre. She has some sort of magic related to seeing the future. It's not a stretch to think she could find a way to time travel in a fanfiction and change the course of events in Westeros.
He would likely interact with his father, Minato. Since Minato was very active during the Third War, Naruto would have a lot to talk to him about, like his own experiences and the future. Another character would be Jiraiya. Jiraiya was also involved in the war and he was Naruto's mentor, so Naruto would surely seek him out. And of course, he might interact with the Third Hokage as well, to gain more knowledge about the past and how to handle the situation in the war.
Iroh would be fascinating to see time - traveling. He has so much wisdom already. If he traveled to different times, he could share his knowledge and perhaps change some key events. He could also gain more insights from other time periods, which would further enrich his already deep understanding of the world.