Their relationship would likely become more complex. At first, they might be more united because they have a common goal of changing the past. However, as they face different situations in the past, their differences could start to surface again. For example, Sasuke might be more focused on revenge - related issues from the past, while Naruto would be more interested in saving lives. This could lead to some arguments between them.
In the time - travel fanfic, their relationship could transform in multiple ways. Initially, they'd be comrades with a shared mission. But as they interact with the past, Sasuke might remember his old grudges more vividly. He could be more withdrawn and brooding, which Naruto might not understand as well as he does in their original time. On the other hand, Naruto could be more eager to make changes that Sasuke might think are too risky. Their relationship would be a tug - of - war between their different personalities and goals, all while navigating the complex world of the past. There could also be moments of great understanding and growth for their relationship, like when they see how their actions in the past can shape their future selves.