Naruto would be a very likely character to interact with Hinata. Since they have a strong connection in the original story, in a time - travel scenario, their relationship could develop in new and interesting ways. Maybe Hinata could be more open with her feelings earlier in time.
Definitely Hagrid. Hagrid has always been around Hogwarts and has a connection to the students. Sirius could interact with Hagrid if he goes back in time. Maybe Hagrid could help Sirius in some way, like providing shelter or information. And Bellatrix Lestrange could also be in the picture. Since she is part of the Black family, Sirius might have to deal with her in the past, either to try to change her or to stop her from doing something evil.
A third plotline could involve Hinata going back in time and having to deal with the consequences of changing the timeline. Her actions might create new problems or enemies. For instance, if she changes a major event, it could disrupt the balance of power among the ninja villages, leading to unforeseen conflicts that she then has to resolve.
Bellatrix Lestrange could also be a character who interacts with Snape. If Snape goes back far enough, he might try to influence Bellatrix's path and keep her from becoming such a devoted Death Eater. This could have a major impact on Voldemort's power and the overall balance of the wizarding world.
His father's friends, the Marauders, are very likely to interact with Harry if he goes back in time. He could become a part of their group, learn their secrets, and even participate in some of their pranks. Hagrid is another character. Harry has always had a special bond with Hagrid, and going back in time, he could strengthen that bond even more. And of course, Snape. Harry could try to change their relationship from the start and understand Snape better.
Often, in fanfiction, Itachi's calm and collected demeanor contrasts with Hinata's more timid nature. This contrast can lead to some interesting dynamics. For instance, Itachi might encourage Hinata to be more assertive, while Hinata could show Itachi the beauty in simple things, like the flowers in Konoha. Their relationship might grow as they learn from each other.
Sakura would probably be a side character. She could be the one who checks on Hinata's health during her pregnancy if that's part of the story. Shikamaru might also be there, helping Naruto with some of the strategic planning for the village while also giving him relationship advice. And Boruto and Himawari would be important side characters as their children, adding a family - centered aspect to the fanfiction.
The Hyuga clan as a whole might also take action. Neji, for example, being Hinata's cousin, would feel responsible for her safety. The Hyuga have their own techniques and strength. They could mobilize their forces to track down Itachi and rescue Hinata, especially since she is an important member of their clan.
Obviously, Naruto and Kurama are going to be main characters. Since it's about Kurama going back in time, he'll be central to the story. Naruto will also be important as Kurama's jinchuriki and the one whose life Kurama is likely to impact. Minato might also be a significant character as he was Naruto's father and had a major role in the original story. And there could be appearances from other key characters like Sasuke, as Naruto's relationship with him was a big part of the original Naruto series.
Gohan will most likely encounter other Z - fighters like Krillin and Tien. They were all part of the group back then. Also, there's a good chance he'll meet Master Roshi. Roshi was a key figure in Goku's training, so his interaction with Gohan could be really interesting. He might also come across Bulma, who was always involved in the Dragon Ball story, either by accident or because he needs her help with some time - related technology or something.
Obviously Naruto would be one of the key characters. Since it's about his generation going back in time, he would be central to any attempts at changing the past. His knowledge and his will to protect his village would drive many of the actions.