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.
There could be many unknown authors writing such a fanfiction. It might be a new and emerging writer in the Naruto fanfiction community who just had this interesting and somewhat out - of - the - box idea. It's really hard to determine a specific author without more information.
One possible plot could be that Kushina has some misunderstanding about Sakura. Maybe she thinks Sakura is being disrespectful to Naruto in some way, so she punches her to teach her a lesson. Then, Sakura tries to figure out why Kushina is so angry and they start a series of interactions that eventually lead to them resolving their differences.
In some fanfictions, Sakura's punch might be part of a comical situation. For example, Naruto says something really stupid and Sakura just can't help but punch him. But this punch then leads to a series of adventures as Naruto tries to get back at her in a friendly way.
I recommend 'The Punch that Changed Everything'. In this fanfiction, Sakura's punch makes Naruto see the error of his ways regarding a certain mission. It's a short but impactful read.
Well, Naruto would probably be in a great deal of pain. He might double over and be unable to move for a while. It could also lead to a big argument between him and Sakura, with Naruto feeling betrayed and Sakura perhaps realizing she overreacted.
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.
It would likely strain their relationship at first. Sakura would probably be very angry and hurt, both physically and emotionally. She might start to question Naruto's true nature and whether he really was the friend she thought he was.