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.
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 Sasuke might become more withdrawn at first, dealing with the enormity of what has happened.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.
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.
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.
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.
Naruto would be devastated. Sasuke is his closest friend and rival. He would likely go into a state of deep mourning at first. He might isolate himself for a while, just trying to come to terms with the loss.
I haven't come across a particular 'naruto dies saving sasuke fanfic' myself. However, you might want to check Naruto - centered forums where fans discuss and share such stories. Maybe by asking around in those communities, you'll find what you're looking for.
Maybe Naruto leaves because he discovers some deep-seated secrets about Konoha that he can't accept. Or it could be that his relationship with Sasuke changes his perspective and he needs to find a new place to grow and evolve.
One possible plot development could be that Sasuke is in a life - threatening situation, perhaps against an extremely powerful enemy. Naruto sacrifices himself to give Sasuke a chance to escape or defeat the foe. This would likely lead to Sasuke being filled with guilt and regret, and might change his whole outlook on life.