The plot might center around the origin of the Katon Gouka Mekkyaku. In the fanfic, it could be revealed that this technique was once used by an ancient and powerful ninja clan. Naruto discovers some hidden secrets about this clan while trying to master the technique better. This could lead him on a journey to find ancient scrolls or talk to old sages who might know more about it. Along the way, he would face various challenges and also learn more about the history of his own village and the ninja world in general.
Well, the plot could involve Naruto exploring new ways to use the Katon Gouka Mekkyaku. Maybe he's in a battle against a powerful enemy who has some sort of resistance to fire techniques. So, Naruto has to combine it with other jutsus or find a new way to channel the chakra for this technique to make it more effective. He could also be teaching the technique to some of the younger ninjas in Konoha, which would lead to some interesting training scenes and character development for those new ninjas.
One possible plot could be that Naruto accidentally unleashes a much more powerful version of the Katon Gouka Mekkyaku during a training session. This causes a lot of chaos in Konoha. As a result, he has to figure out how to control this new power. He might seek help from his friends like Sasuke or Sakura. They could try different methods, like using medical ninjutsu to balance his chakra or Sasuke's sharingan to analyze the technique better. Meanwhile, there could be some villains who are interested in this new power of Naruto's and try to steal it for their own evil purposes.
Chapter 733, "Those People from the Jiang Family Are More Stupid than the Next!", is the clearest early answer because it turns Next's situation into a concrete plot event. The chapter turns that into something concrete by moving from Okay, Sister, you should go to sleep now.
Chapter 670, "Sister Will Support You", is the clearest early answer because it turns Bai Lu's situation into a concrete plot event. The chapter gives readers a real answer by opening on Jiang Li, sensing that something might have gone wrong, quickly used...
Chapter 485, "Straight On Clash!", is the clearest early answer because the buildup finally cashes out in a real clash.
Chapter 776, "Sister Li Keeps a Toy Boy?", is the clearest early answer because it turns Toy Boy's situation into a concrete plot event.
Chapter 813, "She Has a Child?!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event.
A practical answer is Chapter 2, because it turns the idea into a concrete scene. The chapter opens with On the top floor of the Financial Building, a gathering of dark clouds..., adds pressure through In my opinion, Mother should kick out this troublemaker who's a stranger., and ends on Jiang.
Chapter 570, "A Simple Auto-Attack Baits Out an Ultimate from the Enemy", is the clearest early answer because the buildup finally cashes out in a real clash.
Chapter 784, "Slander Brother Yang Without Any Proof?!", is the clearest early answer because it turns Mr's situation into a concrete plot event.
Chapter 734, "Are These Primary School Students Arguing?", is the clearest early answer because it turns Brother Jiang's situation into a concrete plot event. The chapter gives readers a real answer by opening on Old Sir Jiang was especially happy.
Chapter 558, "The Love-Struck, Flirty Wife Appears", is the clearest early answer because it turns Luo Xuan's situation into a concrete plot event. The setup is These people's comments are just too unpleasant;