Naruto might encounter a group of rogue ninjas. At first, they seem like enemies, but as he gets to know them, he realizes they have their own reasons for being outcasts. They could teach Naruto some unorthodox fighting styles that are not taught in the Leaf Village. Also, there could be a mysterious traveler who has knowledge about Naruto's family that even he doesn't know. This traveler could be the key to unlocking some hidden power within Naruto.
He could feel a sense of guilt. Sasuke might think that his actions or their relationship issues pushed Naruto to leave. He may then set out to find Naruto, not only to bring him back but also to resolve their long - standing issues. For example, if there were unspoken words or unresolved fights between them, Sasuke would see this as an opportunity to set things right.
The encounter Naruto has after leaving the forest is also important. It could be with a new enemy, a potential ally, or even a long - lost friend. And of course, Naruto's growth as a character. He might learn new jutsu or gain a new perspective on life outside the forest.
Well, Naruto could leave the village in a fanfic due to a prophecy. He might believe that leaving is the only way to fulfill a greater destiny that the village can't support. As he leaves, he starts to explore different cultures of the ninja world. He learns about new forms of jutsu and how other villages operate. He could also form a team with outcasts from different villages, and together they strive to change the way the ninja world views things.
It could also be that Naruto is forced to leave Sakura due to some external threat or danger. For instance, if there is an enemy targeting Naruto and he fears that being around Sakura will put her in harm's way, he would choose to leave to keep her safe. This is in line with Naruto's selfless nature as seen throughout the series.
One possible reason could be that Naruto feels he has learned all he can from Jiraiya at that point. Maybe he wants to explore his own path independently and test his skills without Jiraiya's constant guidance.
Maybe he marries an OC (original character). The author of the fanfic could create a whole new character with unique qualities that attract Naruto during his time away from Konoha.
Another possibility is that Naruto marries a completely new character. This character could be from a far - off land that Naruto visits during his departure from Konoha. She could be a powerful ninja in her own right, with unique abilities and a personality that attracts Naruto, leading to their marriage.
In a 'naruto and kushina leaves konoha fanfic', perhaps they leave due to a threat to Naruto's safety that the village can't fully protect against. Maybe some secret organization is after him and they feel they have no choice but to go into hiding outside Konoha.
Definitely Naruto and Sasuke. Naruto is the one leaving the village, so he's a central figure. Sasuke, given the 'narusasu' aspect, is bound to play a crucial role too. He could be the one Naruto is leaving behind, or he could be the one who follows Naruto out of the village. There might also be other supporting characters from the Naruto universe like Kakashi or Sakura who could be affected by Naruto's departure.