She might be shocked at first. Sakura has always known Naruto as someone who is very attached to Konoha. If his leaving is because of her, she could feel guilty. She might try to stop him, asking him why he's doing this and telling him that he doesn't have to go.
Well, it could be that Naruto feels he doesn't fit in Konoha anymore. Maybe he's constantly being misjudged by the villagers despite his efforts to protect them. Also, if his relationship with Sakura is a big factor in his leaving, it might be that he can't bear to see her with someone else, like Sasuke perhaps, and decides to leave to avoid the heartache.
I'm not sure about a specific one off the top of my head, but you can try searching on fanfic platforms like Archive of Our Own. Just type in 'Sakura leaves Konoha' and you'll likely find some great ones.
Maybe there are some dark secrets within Konoha that he discovers in the fanfic. These secrets could be related to the treatment of certain individuals or something more sinister about the village's leadership. Discovering such things could make Naruto decide to leave in order to distance himself from the corruption or unfairness.
Sasuke might initially be shocked. He has always seen Sakura as someone connected to Konoha, so her leaving would be unexpected. He could be concerned about her safety, especially if she leaves without proper preparation or protection.
Sakura would probably be very worried. She has always cared about Naruto deep down, so she might be constantly trying to find a way to bring him back. Kakashi, on the other hand, would likely understand Naruto's reasons to some extent. He might be sad but also respect Naruto's decision. As for Sasuke, if he is in the village at the time, he could be conflicted. He and Naruto have a complex relationship, and Naruto's leaving might make him realize how much Naruto actually means to him.
One possible reason could be for Naruto's safety. Maybe she has to go on a dangerous mission and believes Konoha is the safest place for him. Another reason might be that she is forced by some powerful enemies who threaten Naruto's life if she doesn't leave him in Konoha.
In a 'naruto and kushina leaves konoha fanfic', perhaps they leave due to a threat to Naruto's safety that the village can't fully protect against. Maybe some secret organization is after him and they feel they have no choice but to go into hiding outside Konoha.
In a 'Naruto Namikaze leaves Konoha' fanfic, it could be that Naruto is framed for a crime he didn't commit. The villagers turn against him, and even his friends start to doubt him. Feeling betrayed and alone, he decides to leave Konoha. As he travels, he meets other outcasts who have also been wrongly accused, and together they form a group to clear their names and seek justice.
One idea could be that as Naruto leaves Konoha, he encounters a powerful and ancient creature in the wild. In a battle with it, the extreme stress and need to survive unlocks his hidden bloodline. This new power could be related to some kind of elemental control that is unique to his clan, like a special form of wind manipulation that is far more powerful than what he had before.