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.
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.
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.
Chapter 623, "Turned into a Little Crybaby", is the clearest early answer because it turns Su Yinwan's situation into a concrete plot event. The chapter gives readers a real answer by opening on Jiang Cheng woke up again, this time realizing he was being carried...
Chapter 379, "Is This the Way Huaxiu Treats Its Guests?", is the clearest early answer because it turns Huaxiu's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Song Man chuckled twice, evidently displeased, and then positioned...
Chapter 379, "Is This the Way Huaxiu Treats Its Guests?", is the clearest early answer because it turns Liu Suqing's situation into a concrete plot event. It opens with Song Man chuckled twice, evidently displeased, and then positioned...
Chapter 656, "If he's the grandson of the richest man, then I am the richest man!", is the clearest early answer because it turns Shang Shuyue's situation into a concrete plot event.
Chapter 482, "Someone Might Need Me", is the clearest early answer because it turns Hua Country's situation into a concrete plot event. It opens with Other people caught a glimpse of Zhao Lang's cold and stern face and...
Chapter 367, "Help Me Call the Police", is the clearest early answer because it turns Princess Disease's situation into a concrete plot event. It opens with However, before they could approach, another loud "bang" sounded as...
Chapter 660, "Includes Meals and Accommodation?", is the clearest early answer because it turns Bodyguard Ji's situation into a concrete plot event.
Chapter 383, "Doomsday Beehive", is the clearest early answer because it turns Shen Lan's situation into a concrete plot event.
Chapter 510, "The Fake Young Master", is the clearest early answer because it turns Gu Mansion's situation into a concrete plot event. Early on, Ever since that night's family banquet, Gu Qiang hadn't slept soundly....
Chapter 380, "From today on, you no longer need to work here", is the clearest early answer because it turns Miss Jiang's situation into a concrete plot event.