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.
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.
One possible plot could be that Sasuke sacrifices himself to save the village. He might face an enemy too powerful for him alone, but he decides to hold them off long enough for Naruto and the others to escape. This would be a very tragic end for Sasuke as he has always been a complex and somewhat brooding character, and his death in this way would show his growth into a selfless hero.
One possible fanfiction could be 'The End of the Uchiha' where Sasuke sacrifices himself to save the village from a powerful dark jutsu. In this story, his death is a heroic and tragic end to his journey.
Sasuke might be in shock at first. He can't believe that Naruto is dying in his arms. He may feel a great sense of guilt, thinking that he could have done more to prevent it.
Another theme could be the cost of power. Maybe Naruto's death protecting Sasuke was related to some powerful force or enemy that was after Sasuke. This shows that the pursuit of power or protecting someone with power can have dire consequences. Emotionally, the story could explore how the characters move on from such a tragic event. Some might use Naruto's death as a motivation to become stronger, while others might be paralyzed by grief.
One possible reason could be that he overestimated his abilities during a battle in the Wave arc. Maybe he took on an enemy that was much stronger than he thought and got fatally injured. Another reason could be that he sacrificed himself to save Naruto or someone else. It's also possible that there was some sort of betrayal within the group they were with, and Sasuke was caught off - guard and killed.
It could be an unknown, ancient evil that has been awakened. This villain might have powers far beyond what Naruto and Sasuke have faced before. Maybe it's an entity from a forgotten era that was sealed away long ago and now has broken free.
One possible scenario could be that they sacrifice themselves to save the village from an ultimate evil. They might combine their powers in a final, desperate move that exhausts them completely, leading to their deaths.