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.
Well, in some versions of this fanfiction, it might be that Naruto uses a powerful and forbidden jutsu to defeat an overwhelming foe. This jutsu drains his life force, and Sasuke, who has just realized how much Naruto means to him, catches Naruto as he collapses. It could also be that Naruto is poisoned, and Sasuke tries his best to save him but fails in the end, and Naruto passes away in his arms.
One emotional moment could be Sasuke's realization of losing Sakura. His usually stoic facade might crumble as he holds her lifeless body, perhaps reminiscing about all their shared moments, from their early days as teammates to the more complex relationship they developed over time.
He could also become more vengeful. If Sakura's death was caused by an enemy, Sasuke would stop at nothing to avenge her. His focus would shift solely to seeking revenge, and his already formidable skills would be honed even further in his quest for retribution.
It could be a consequence of Naruto's own overuse of the Nine - Tails' chakra. Naruto has always been pushing his limits when it comes to using the Nine - Tails' power. If he overdid it in a crucial battle, it could have damaged his body beyond repair. Kakashi, as his former teacher and a trusted friend, would be the one to hold him as he passed away, mourning the loss of such a great hero.
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.
Well, Naruto's death would likely shatter Sasuke. He has always had Naruto as a sort of anchor, whether he admitted it or not. His whole life has been intertwined with Naruto's. Naruto's death might make Sasuke question his entire purpose. He could initially be filled with rage and a thirst for revenge, thinking that he could have prevented it. This could lead him down a darker path at first. But eventually, it might also make him realize the importance of Naruto's ideals and he could strive to carry them on in Naruto's memory.
Kakashi's reaction would be complex. He is a man who has lost many people in his life. But Naruto was special. He would hold Naruto tightly, his hands trembling slightly. He might try to use some of his own chakra to keep Naruto alive a little longer, but deep down he knows it's futile. His face would show a mixture of pain, regret, and love. He would remember all the times he trained Naruto, all the lessons he taught him, and how Naruto had surpassed all expectations.
Well, if Sasuke dies in this fanfiction set in the Wave arc, Naruto would likely become more determined to avenge him. He might also grow more solemn and focused on his ninja training, realizing the fragility of life. His view of the world could shift from being a bit more carefree to a darker, more serious one as he grapples with the loss of his friend and rival.
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.