It could be Orochimaru. He might be interested in Sakura's potential as a medic - ninja and wants to use her for his experiments. Naruto would then have to face Orochimaru once again to rescue Sakura and bring her home.
In this fanfiction, perhaps Sakura is kidnapped by a group who wants to use her medical skills for their evil purposes. Naruto quickly sets out to rescue her. He uses his newfound powers and the help of his friends like Kakashi and Shikamaru. When he finds Sakura, she is weak but still defiant. Naruto has to fight the kidnappers, and his determination to bring Sakura back home gives him the extra strength he needs to defeat them all.
Their conversation could also be more personal. Naruto might ask Sakura about her family, and Sakura could share some heartwarming stories. She might also ask Naruto about his time away from the village on his long missions. And Naruto would tell her all the interesting things he saw and the people he met. There could be some light - hearted banter in between, like Naruto teasing Sakura about her temper.
One possible plot detail could be that they start off by reminiscing about their old adventures during the walk. Naruto might crack some of his usual jokes to make Sakura laugh. They could also talk about their friends and how everyone has changed over time.
In some fanfictions, Naruto might be influenced by an external force. Maybe a villain has the ability to manipulate emotions, and they target Naruto, making him lose control and hit Sakura back. This then sets up a story arc where Naruto has to overcome this external influence and make amends with Sakura.
Perhaps there was some sort of dark influence at play. In the world of Naruto, there are often dark forces and genjutsu (illusion techniques). It's possible that Naruto was under the influence of a genjutsu that made him see Sakura as a threat, so he punched her in self - defense without realizing it was an illusion.
On Naruto's side, he would probably feel guilty after realizing what he had done. This guilt could make him try to make amends, but Sakura's initial reaction of anger and hurt would still be a big obstacle in their relationship. Their interactions would be filled with tension and awkwardness.
One possible story could be that Naruto accidentally hits Sakura back during a sparring session. Maybe he was trying a new jutsu and misjudged the distance. Another could be in an alternate universe where Naruto is under a spell that makes him act out of character and he hits Sakura back when she tries to stop him from doing something dangerous.
It could become more strained at first. Sakura would be hurt, both physically and emotionally, and she might hold a grudge against Naruto. However, if Naruto had a good reason for punching her, like protecting her from a greater danger she didn't see, she might eventually come to understand and their relationship could become stronger in the long run.