Perhaps an ancient evil entity that was sealed away long ago in the Land of Fire. Naruto's rise as the Lord awakens this entity, which sees the land as its rightful domain. It uses dark jutsu and mind - control on some of the villagers to create chaos and tries to drive Naruto out so it can rule over the land once again.
One possible main antagonist could be a rogue ninja from a forgotten clan who has a vendetta against the Land of Fire. They might be seeking revenge for something that happened in the past and see Naruto as the symbol of the new regime they want to topple.
It could be a group of corrupt politicians within the Land of Fire itself. They oppose Naruto's leadership because they fear that his reforms will cut into their own power and wealth. So they plot behind the scenes to undermine him at every turn, and they might even hire mercenary ninjas to carry out their dirty work.
Chapter 562, "So You Are Not First, But Last?!", is the clearest early answer because it turns Ai Xiaochun's situation into a concrete plot event. A useful checkpoint is the way the scene starts with After they finished preparing the fish, Guo Xing and Luo Xuan got...
Chapter 907, "Disgusting Clothes", is the clearest early answer because it turns Disgusting Clothes Zhao's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Zhao Lang originally thought this little kid was just casually saying...
Chapter 812, "You’re Extorting Money!", is the clearest early answer because it turns Jiang Mingchuan's situation into a concrete plot event.
Chapter 715, "Childhood Shadow", is the clearest early answer because it turns Childhood Shadow Jiang's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Jiang Mingyu let out a long sigh before collapsing onto the sofa,...
Chapter 584, "The Wealthy Young Master Renounces Worldly Life for Love", is the clearest early answer because it turns Life's situation into a concrete plot event. Early on, There wasn't a mirror on the small boat..
Chapter 452, "Party of Four", is the clearest early answer because it turns Party's situation into a concrete plot event.
Chapter 568, "Not at All", is the clearest early answer because it turns Crown Prince's situation into a concrete plot event.
Chapter 374, "From Now On, Jiang Li Calls the Shots in Golden Sand!", is the clearest early answer because it turns Gu Yu's situation into a concrete plot event. The chapter turns that into something concrete by moving from Jiang Li's bold behavior infuriated Jin Siyue, but it also succeeded...
Chapter 460, "Saving People", is the clearest early answer because it turns Saving People's situation into a concrete plot event. The chapter gives readers a real answer by opening on [HAHAHAHAHAHAHA, Jiang Yan is impersonating me.] [At this moment, I...
Chapter 763, "It must be Jiang Li who tied her up!", is the clearest early answer because it turns Shang Shaojing's situation into a concrete plot event.