One possible plot could be that Naruto has faced so many betrayals. Maybe his friends turn on him due to some false accusations. This shatters his warm - hearted nature, and he becomes cold as a defense mechanism. He starts to isolate himself and no longer trusts easily.
One possibility is that Naruto might feel a sense of isolation. Since Sasuke has achieved the highest position in the village, Naruto might feel like his own journey and achievements are overshadowed. He could become cold as he withdraws into himself, thinking that he no longer has a clear purpose in the village where his best friend now holds all the power.
If Naruto becomes cold, his relationship with Hinata would be severely affected. Hinata, who loves him for his warm heart, might be at a loss. Naruto might start to push her away, not allowing her to get close. His friends from Team 7 might try to bring back the old Naruto, but his coldness would create a rift between them.
Naruto could become cold due to disappointment. He might have expected Sasuke to rule in a certain way, but when Sasuke becomes Hokage and starts making decisions that seem contrary to Naruto's ideals, Naruto feels let down. This disappointment could turn him cold towards Sasuke and the village in general.
One possible plot could be that Naruto experiences a great betrayal, perhaps by someone he deeply trusted in the village. This could make him lose faith in the bonds he once cherished and become cold towards others. He might start to isolate himself and focus only on his own power, believing that relying on others will only lead to more pain.
Most Naruto fanfics tend to focus on his adventures, his relationships in a positive way, like his bond with Sasuke or his training with Kakashi. The idea of Naruto becoming a pervert is quite out of the ordinary for the Naruto - themed fanfic world. It's not something that is typically explored in mainstream Naruto fan creations.
Maybe Percy has been through a great betrayal, like his closest friend turned on him for power. So he starts to close off his emotions to avoid being hurt again.
His relationship with Sakura would definitely be affected. Sakura, who has always known the warm - hearted Naruto, would be shocked and confused. She might try to reach out to him at first, but if he keeps pushing her away, she could become frustrated or even give up. Their friendship would be strained.
One possibility could be a great tragedy that befalls him. For example, if all his friends were brutally killed in front of him by an enemy he couldn't defeat at that time. This might shatter his belief in the goodness of the world and drive him to become cold - blooded in order to seek revenge. He would start to see killing as the only way to get justice.
One possible plot could be that after losing many friends in a major battle, Naruto becomes disillusioned. He might start to see the world as a cruel place where strength is the only thing that matters. So he shuts off his emotions to avoid further pain, and becomes cold - hearted towards his former allies and enemies alike.