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.
Perhaps in the fanfiction, there is a plot where Naruto is tricked into a dangerous situation by a cunning villain. He endures great harm trying to protect Sasuke and the village. By the time Sasuke arrives to help, Naruto is on the verge of death. Sasuke holds Naruto as his life slips away, and this event could be a turning point for Sasuke, making him more determined to protect the things Naruto cared about.
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.
Perhaps in the fanfiction, there's a curse that was meant for Snape but Harry jumps in to take it. As Harry is dying in Snape's arms, Snape has a moment of deep reflection. He might think about all the times he could have been kinder to Harry, and how Harry has now become a symbol of the good that Snape had long forgotten in his own life. And this moment of Harry's death could be a turning point for Snape's character development in the fanfiction.
In certain 'Bella dies in Edward's arms' fanfictions, it might be related to time - travel elements. Imagine a situation where Bella travels back in time to change something for the better in Edward's past, but in doing so, she disrupts the timeline in a way that causes her own death. When she returns to the present (or what was the present before the time - travel), she is already dying and Edward holds her as she passes away. Another idea could be that Bella's human frailty, combined with the dangerous vampire world she's a part of, leads to her being in a situation where she can't be saved, and Edward has to watch her die in his arms.
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.
Perhaps in the fanfiction, Naruto is betrayed during the battle. By someone he trusted, like a comrade who was secretly working for the enemy. This betrayal catches him off - guard, and he is severely injured. Unable to fully recover, he succumbs to his wounds during the fight.
One possible scenario could be that Naruto was severely injured in a battle, perhaps against a powerful new enemy. He was brought to the hospital, but his injuries were too grave to be healed, and he passed away surrounded by his friends and family, leaving everyone in deep sorrow.
Another scenario could be that they are poisoned by a new, undetectable toxin. As they gradually weaken, they try to find a cure but run out of time. Their deaths would be a great loss, and it could lead to major changes in the political and social structure of the ninja world as everyone copes with their passing.
One scenario might be related to a failed mission. Maybe Naruto is sent on a top - secret mission that is much more dangerous than anyone anticipated. He could be betrayed by someone he trusts during the mission, leading to his death. Or perhaps there are some unforeseen circumstances, like a strange and deadly curse that gets unleashed during the mission which he succumbs to.
Perhaps in this fanfiction, Naruto is fatally injured by a powerful enemy who has lured him to the Valley of the End. He's outnumbered and despite his best efforts, he succumbs to his wounds. His death then has a profound impact on all his friends and the entire ninja community, leading to a new era of unrest and a search for a new hero.
In the fanfic, it could be that Nico has some sort of internal struggle, like a dark magic within him that he can't control. He asks Percy to be there when he tries to fight it one last time. But the magic overcomes him and he dies in Percy's arms. This would be a very tragic and emotional moment, with Percy left to wonder if he could have done more to save Nico.