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.
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.
Well, one possible consequence could be that Naruto is seriously injured. Maybe he has a broken bone or gets knocked out cold. It could also lead to some sort of rift between them, with Naruto feeling betrayed by Sakura's overly aggressive action.
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.
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.
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.
Sakura can be quite emotional in these fanfics. She may cry when thinking about how she let Naruto go or when she faces difficulties in getting him back. But she also uses these emotions to fuel her determination. She'll often pour her heart out to Naruto when she finally gets the chance to talk to him.
Well, one way could be that Sasuke realizes how much Sakura has always loved him. He might come back after a long journey of self - discovery. He could show up at the village where Sakura is, looking all brooding and mysterious as usual. Then he'd just say something really simple like 'I'm back, Sakura' and she'd be so happy she'd probably cry. And that could be the start of them getting back together in the fanfiction world.