One possible storyline could be that Naruto sacrifices himself in a final battle against a powerful enemy who is also after Sasuke. His death then makes Sasuke realize the true extent of Naruto's friendship and he decides to fully embrace the path of good. Another could be that Naruto uses some sort of ultimate jutsu that drains his life force in order to break the dark influence on Sasuke and bring him back to the village. And finally, Naruto might die protecting Sasuke from an attack that was meant for Sasuke, which shocks Sasuke into changing his ways.
Sasuke's reaction would be the most complex. He would be filled with guilt for causing Naruto's death. He would also feel a new sense of purpose, to live up to Naruto's sacrifice. Sasuke might become more withdrawn at first, dealing with the enormity of what has happened. The other ninja from Naruto's generation, like Shikamaru, would be sad but also respect Naruto's choice. They would support Sasuke in his new path, as they understand it was what Naruto wanted. And the younger generation would see Naruto as a martyr, someone who sacrificed for a greater good.
One possible storyline could be that in a major battle against a powerful enemy, Hinata jumps in front of Naruto to take a fatal blow meant for him. This could lead to Naruto going into a rage and unlocking a new level of power in his grief. He might then defeat the enemy single - handedly, vowing to carry on Hinata's will.
A possible plotline is that Sasuke is being held captive in a different dimension. Naruto has to find a way to enter this dimension. He meets some strange creatures there who can either help or hinder his mission. Naruto uses his skills and wits to overcome the obstacles. Once he finds Sasuke, Sasuke is initially resistant to coming back. Naruto then has to convince him by reminding him of their dreams when they were kids, like changing the shinobi world for the better. Naruto's unwavering belief in Sasuke eventually breaks through Sasuke's defenses and they return together.
In some fanfics, it could be a situation where they are trying to save the ninja world from a great disaster. They end up using a powerful jutsu that requires both of their life forces. They know that this jutsu will kill them, but they are willing to sacrifice themselves for the greater good. As they perform the jutsu, they share memories of their past, their rivalry, and their friendship, and then they die together, leaving behind a world that they have saved.
In a fanfiction of Naruto dying of a sickness, perhaps the sickness is a result of overusing his chakra during a great battle. His body starts to break down, and he begins to experience weakness and fainting spells. Hinata, being his wife, is desperate to find a way to save him. She might turn to her own clan's secrets in hopes of finding a cure. At the same time, Naruto tries to train Boruto in what little time he has left, passing on his values and skills, while also coming to terms with the fact that he won't be there to see his son grow up fully.
In a NaruSaku - Naruto brings Sasuke back fanfiction, Naruto could have a vision of a future where Sasuke's return and his relationship with Sakura are intertwined. This vision gives him the strength to endure all the challenges on his way to find Sasuke. Sakura, on the other hand, is constantly worried about Naruto but also has faith in him. When Naruto finally reaches Sasuke, he tells him about this vision and how their lives are all connected in Konoha, which makes Sasuke start to consider coming back.
Maybe Naruto is affected by a dark and ancient curse. This curse was placed on him by an unknown enemy. It starts to take hold gradually, making him weak and sick. He begins to have strange visions and nightmares. His body slowly deteriorates, and he loses his appetite. His friends notice his strange behavior and the physical changes, but Naruto is determined to keep fighting until they find out what's wrong with him.
One possible storyline could be the child's struggle with living up to the legacies of their parents. Sasuke and Sakura are both very powerful ninja, and the child might feel pressured to be just as great. Another could be the child making their own unique mark in the ninja world, perhaps by developing a new jutsu or forging new alliances.
Well, it could go in many directions. Maybe Naruto starts a quest to find a way to bring peace to the ninja world on his own terms. Or perhaps he has to deal with the consequences of leaving Konoha and tries to prove his intentions were pure.
Well, it could be that Naruto leaves because he realizes that Konoha's ways don't align with his ideals anymore after that intense mission. Or perhaps there are hidden threats that force him to go away for the greater good.