It could be Orochimaru. He might be jealous of their new merged power and see it as a threat. So, he tries to find a way to either steal it or destroy them.
Madara Uchiha could be a great choice for the main villain. Even if he's been defeated before, in this fanfiction, he could be resurrected with new powers. He would see the Naruto - Sasuke merge as an obstacle to his plan of reshaping the ninja world according to his will.
The main villain could be a new character. Maybe a rogue ninja who has been researching the secrets of power fusion for years. He sees Naruto and Sasuke's merge as the perfect specimen to study and will stop at nothing to capture them, even if it means destroying the entire ninja world in the process.
A practical answer is Chapter 160, because it turns the idea into a concrete scene. The chapter opens with As the sky gradually darkened, the originally mild drizzle began to..., adds pressure through Feeling somewhat guilty, she bluffed, "I—I like her, it's none of—none of..., and ends on Then.
A practical answer is Chapter 135, because it turns the idea into a concrete scene. The chapter opens with The sun blazed down mercilessly, but thankfully, the path to the mountain..., adds pressure through At this moment, it seemed more likely to be grim than not., and ends on Are you healed from.
If you want the clearest chapter reference, go to Chapter 151. Early on, Now it was Zhang Cheng's turn to be confused.; later, the chapter leans on The problem lies with He Yun?. By the close, I will have my assistant pay the bill., and the chapter closes with Was he...
Chapter 157 is the strongest chapter-level answer because it starts from It was already the last day of Flash Change in his dream. and follows through on I'm laughing myself crazy, do actors all have such strong conviction?]....
A practical answer is Chapter 148, because it turns the idea into a concrete scene. The chapter opens with Exiting the cottage, Director Sun picked up the phone., adds pressure through Two irrational people might indeed be able to turn the tide., and ends on Just as Jiang Li put down her.
A practical answer is Chapter 205, because it turns the idea into a concrete scene. The chapter opens with Ah Fu had no idea what the video Jiang Li mentioned was., adds pressure through Let's play together from now on., and ends on Having such a sister certainly would bring happiness..
A practical answer is Chapter 159, because it turns the idea into a concrete scene. The chapter opens with Seeing the red light of the camera turn on, Su Yinwan looked at the..., adds pressure through The moment the door closed, he straightened his back, and his originally..., and ends on Shang.
Look to Chapter 165 for the clearest answer. It begins with How come everyone else got a solid six points, and he ended up with just..., keeps building through Director Sun spread his hands, "Our staff calculated the total score with..., and lands on Various explosive Weibos sprang up like.
A practical answer is Chapter 159, because it turns the idea into a concrete scene. The chapter opens with Seeing the red light of the camera turn on, Su Yinwan looked at the..., adds pressure through The moment the door closed, he straightened his back, and his originally..., and ends on Shang.
Chapter 152 is the strongest chapter-level answer because it starts from While most of the villagers of Chixia Village were gathered at the village... and follows through on That shouldn't be." Zhao Lang's eye color darkened, "I'm afraid he's....