There could be historical ponies. For example, the founders of Equestria or ancient pony scholars who knew a great deal about magic. These characters could provide Twilight with important information or pose challenges to her. Celestia and Luna in their younger forms might also make an appearance. They could be key to Twilight's mission, either as allies or as characters whose actions Twilight needs to influence.
Obviously, Twilight Sparkle herself. And her friends like Rainbow Dash, Pinkie Pie, Fluttershy, Rarity, and Applejack might also be involved. Maybe they follow Twilight back in time or she has to work with past versions of them.
One possibility is that Twilight Sparkle goes back in time to a crucial moment in the history of magic in Equestria. She may have to ensure that a powerful magical artifact is discovered or protected. This could involve interacting with ancient ponies, solving mysteries, and facing challenges that were long forgotten in the present time. She might even discover secrets about the origin of her own magic powers while on this time - traveling adventure.
Obviously, Yusei himself. And his friends from his original time period might also show up in flashbacks or if he somehow brings them back in time with him. For example, Jack could be there.
Obviously, Josuke himself would be the main character. His friends like Okuyasu could also be involved. Maybe Okuyasu somehow follows Josuke back in time or Josuke has to find a way to bring him back if he gets left behind in the past.
Sokka would also be a common character. His strategic mind could be very useful in a different time period. He might be the one who figures out how to deal with the new situations they encounter while in the past. For instance, if they need to outwit an enemy in a historical battle, Sokka would be the one to come up with a plan.
In addition to the main characters, there could be some minor characters from the manga. For instance, the teachers at UA like Aizawa. Aizawa might notice that something is different about Deku and start to investigate. There could also be villains from the past that Deku has to face again, but this time with his future knowledge. Characters like Stain could be interesting to include as Deku might try to change the way he dealt with Stain in the original timeline.
Dean Winchester is often a common character. Since he has a strong bond with Castiel, he might be involved in the time - travel events either directly or through Castiel's actions in the past that affect him in the present.
There are several potential characters. Aizawa could be one. Godlike Izuku could show up in the past and Aizawa might be suspicious of his powers at first but then come to realize his good intentions. Gran Torino is also a possibility. Izuku could learn different things from him in the past. And then there are the other pro - heroes. They might see Izuku as a strange new force but also recognize his potential to do good. His interactions with them could change the course of hero history in this fanfiction.
One potential plot could be Bloom going back to the time when the first great war in the magical world happened. She could try to stop the war from starting, which would change the entire history of the magical world. This would also mean that the relationships between different magical species would be completely different in the present.
A likely candidate could be Discord. He is known for causing chaos and mischief. He might pretend to befriend Twilight Sparkle only to betray her at a crucial moment. Another possibility is Trixie. Trixie has been jealous of Twilight in the past and could betray her out of envy.
The people who in the future are his enemies but in the past are just normal people might become his allies. For instance, if there is a character who later turns against him, in the past he might be able to befriend that person and change their future path, so they end up helping him in his time - traveling adventures.