In addition to Sasuke, Kakashi, and Sakura, other characters like Iruka sensei would be affected. Iruka was one of the first people to believe in Naruto. Naruto's death would be a huge blow to him. Also, the villagers of Konoha who had started to see Naruto in a new light might have to re - evaluate their views. And characters in the Land of Waves, like Tazuna and his family, who had come to rely on Naruto's help, would also be impacted. They might lose hope or face greater danger without Naruto there to protect them.
Since I don't know the specific fanfic, Sakura is likely one of the main characters as it's in the title. There may also be characters related to her, like her friends or family who are affected by her death in the wave.
Sasuke would definitely be one of the main characters affected. He and Naruto had a complex relationship, and Naruto's death would likely trigger a range of emotions in Sasuke, from grief to anger. Sakura too, as she cared deeply for Naruto. Also, Kakashi, who was Naruto's teacher, would be deeply affected. He had seen Naruto grow and would feel a great sense of loss.
In a 'naruto wave kage fanfic', Naruto is almost always a main character due to his significance in the Naruto universe. The Kage are also central as they represent the leadership of their respective villages. The Wave Village might introduce characters like Tazuna or Inari as main characters too. Tazuna was important in the original Wave arc in Naruto. His skills as a bridge - builder could play a different role in the fanfic, like being a symbol of hope for the Wave Village in a time of conflict involving the Kage. Inari, with his growth and potential, could also have a major part in the story, perhaps forming an unexpected bond with one of the Kage.
I'm not sure specifically as there could be many different versions of such a fanfic. But generally, it might involve Naruto facing some overwhelming danger in the Wave arc that leads to his death, perhaps a more powerful enemy than in the original story or a failed mission that costs him his life.
Family members, if he has any in the fanfiction world, would be devastated. His mother, for instance, would have to deal with the loss of her son. She might become more involved in the affairs of the demigod world, trying to make sure that her son's sacrifice was not in vain. Also, his half - siblings, if any, would be affected. They could be inspired to step up and prove themselves worthy of Percy's sacrifice.
Peter Parker would be extremely affected too. Iron Man was like a mentor to him. His death could make Peter question his own abilities and whether he can continue to be a hero without Iron Man's guidance. He might also feel a sense of guilt, thinking that if he had been stronger or more experienced, he could have saved Iron Man.
Since I haven't read it, I can't say for sure. But it could be Naruto, as he is the main character of the original series and has a lot of pain in his past. It might also focus on Sasuke, who has his own set of traumas and pain related to his family and his quest for revenge.
Twilight Sparkle would definitely be one of the main characters affected. As Spike was her assistant and friend, his death would leave a huge void in her life.
Obviously Naruto would be one of the main characters. And probably characters from the Wave like Tazuna would also be important as Naruto is ruling in the Wave.
Obviously, his close friends like Sasuke and Sakura would be likely to be affected. Since they are often around Naruto, they would be the first ones to experience the nightmares he creates.