They might try to change some past events. For example, Naruto could prevent some of the early tragedies in the village. Hinata might also get the chance to be more assertive earlier in her life, since she has Naruto by her side this time. They could also have an impact on the relationships between different ninja clans.
In this fanfiction scenario, Naruto and Hinata might face a lot of challenges while stuck in the box. Maybe they'd first try to find a way to communicate with the outside world. Naruto, with his never - give - up attitude, would likely start looking for any weak points in the box to break through. And Hinata, being calm and observant, could be searching for any hidden mechanisms or clues within the box.
Their relationship could develop more quickly. In the past, without all the distractions and responsibilities of their original time, they could focus more on each other. Hinata might be more confident in expressing her feelings earlier, and Naruto would likely be more receptive since he's away from his normal hectic life.
Another thing that could occur is that they encounter some strange creatures or phenomena inside the box. Naruto would be ready to fight, but Hinata might first try to understand the nature of these things. They'd have to combine their different approaches to deal with the situation, and in the process, their relationship would evolve, whether it's becoming better friends or something more romantic.
They could use their ninja skills to try and break out of the box. Naruto might try his Rasengan, while Hinata could use her Gentle Fist to find weak points in the box's structure.
Since it's a fanfiction, it could be really diverse in its plot. It may center around Hinata's family history in the past and how that affects her in the Naruto story. Maybe she discovers something about her clan's past that gives her new powers or a new understanding of her place in the ninja world. It could also involve her going back in time to prevent some sort of disaster that was hinted at in the original Naruto series.
Well, if Naruto and Hinata go to the past in fanfiction, it could be really interesting. They could meet their younger selves. Naruto might see his lonely childhood self and try to give him some hope. Hinata could also see her shy younger self and perhaps encourage her to be more confident earlier. And they could also influence the relationships between other characters. For instance, they might help their friends' parents get together or something like that.
Based on the title, Hinata is surely one of the main characters. And since it's related to Naruto, Naruto himself is likely to be an important character as well. There could also be other characters from the Naruto universe, like characters from Hinata's clan such as her father or sister, who might play significant roles in the story as it involves Hinata's journey to the past.