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.
In this 'josuke goes back in time fanfiction', aside from Josuke, the local townspeople from different eras could be involved. There could be an old wise man in the past who helps Josuke understand the rules of time travel. And characters from other Stand - using groups might show up. They could either be allies or enemies. For example, some members of a rival Stand - using gang could cross paths with Josuke in the past and create some tense situations.
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.
The plot of 'josuke goes back in time fanfiction' could be really interesting. Josuke might go back to a time when his Stand ability was just emerging. He could use his knowledge from the future to better understand and control his power earlier. He may also get involved in historical events related to his family or the town he lives in. He could try to change things for the better, but then he has to face the challenges of time travel, like not being able to change some things that are fated to happen no matter what he does.
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, Kaneki himself would be the central character. But also, his friends like Hide could be involved. Hide was always there for Kaneki, and if Kaneki goes back in time, he might want to protect Hide better from the start. Touka could also be a major part. Maybe Kaneki goes back to a time when he first met Touka and tries to change their relationship for the better, and together they could work towards changing the fate of ghouls.
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.
Obviously, Severus Snape is the central character. He is the one who travels back in time, so all the actions and changes are centered around him. But other important characters often include Lily Evans. Since Snape had a deep connection with her, he might try to interact with her in different ways to change the past.
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.
The main characters are Sam and Dean Winchester, of course. But in some fanfictions, Bobby Singer might also be involved. Since he was like a father figure to them, he could be the one guiding them through the time - travel process, or he could be there in the past waiting for them. Also, the characters of their parents, Mary and John Winchester, might show up if they go back to a time when their parents were alive.
Neville Longbottom can also be a main character in some 'Harry Potter goes back in time' fanfictions. Given his growth and importance in the original series, in the fanfic, Harry might go back and help Neville become more confident earlier. This could have a significant impact on the fight against Voldemort as Neville could then be an even more powerful ally from an earlier time.
A young and inexperienced member of the vanguard could also be a main character. Their naivete in dealing with the past could create interesting situations and conflicts. For example, they might accidentally change a small detail that has big consequences.