Another reason could be to bring peace to other regions. Naruto has always been about peace. If he hears of wars or unrest in lands outside the Elemental Nations, his sense of justice might drive him to leave his homeland and try to make a difference there.
One possible reason could be to search for a new power source. Maybe in the fanfic, the powers within the Elemental Nations have become stagnant for him, and he believes there are greater sources of power elsewhere.
One reason could be to seek new challenges. In some fanfictions, Naruto might feel that he has accomplished all he can in the Elemental Nations. He has protected his home, become Hokage, and now he wants to explore the wider ninja world or even other realms that are unknown to the Elemental Nations.
One reason could be to seek new challenges. The elemental countries might have become too familiar, and he wants to explore uncharted regions to test his skills further. Another could be to escape from the fame and expectations in the elemental countries. He might feel suffocated by the constant attention and the high standards set for him.
Sasuke would likely understand Naruto's motives better than most. He himself has had his own reasons for leaving in the past. He might give Naruto some advice based on his own experiences, like how to survive in unknown lands. But at the same time, he would also be a bit envious of Naruto's new adventure.
One possible reason could be to seek new challenges. After achieving so much in the Elemental Nations, he might feel the need to explore areas where his skills can be tested in different ways.
The plot might involve Naruto returning to discover that the elemental nations have changed a great deal in his absence. He could face challenges in terms of politics and power struggles among the different villages, and he'd have to use his skills and influence to restore balance.
It could also be that Naruto has been tricked. For instance, someone he trusts feeds him false information about the elemental nations being involved in a heinous conspiracy against him or his loved ones. This misinformation leads him to take drastic actions and start to destroy the elemental nations. Another idea is that Naruto loses control of his newfound and extremely powerful abilities. These abilities might be too much for him to handle and they end up causing destruction on a massive scale, starting with the elemental nations.
One reason could be that the writer has outgrown the Naruto fandom. As people grow and change, their interests shift. They might find other anime or stories more appealing now.
Perhaps an old enemy that Naruto thought was defeated has returned, stronger than ever. This enemy could hold a grudge against Naruto and the elemental nations for past defeats and wants revenge.
One possible reason could be a conflict within the team. Maybe Naruto felt misunderstood by Sasuke or Sakura. Another reason might be a mission gone wrong where Naruto blames himself and decides he's a liability to the team. Or perhaps he gets an offer from another group that he believes can help him achieve his goals faster.
One possible reason could be for his safety. Konoha is a well - protected village with many skilled ninja. She might believe that leaving him there would ensure he has the best chance at survival and growth as a ninja. Also, she may have some mission or obligation that forces her to leave him behind, perhaps something related to the greater good of the village or the ninja world in general.