Maybe in this fanfiction, Naruto is betrayed by those he trusts the most. His friends turn on him for some false accusations. He tries to fight for his innocence but no one believes him. The loneliness and the pain of being betrayed by his loved ones are too much for him. His spirit is crushed, and his heart can't take it anymore, leading to his death.
Well, one possible plot could be that Naruto loses Sasuke forever. Sasuke might die in a battle against a powerful enemy, and Naruto, who has always considered Sasuke his closest friend and rival, just can't bear the loss. His heart is so broken that he slowly fades away. Another could be that Hinata dies, and Naruto realizes he never truly showed her how much he loved her, and the guilt and grief break his heart.
Sakura would be devastated. She has been Naruto's teammate for a long time. She might blame herself for not being able to save Naruto. She would probably become more withdrawn and focus all her energy on finding a way to bring Naruto back, even if it's impossible. Kakashi would be deeply saddened too. He has always seen Naruto as a special student. He would likely feel that he has failed as a teacher.
It could also be part of a more complex plot where Naruto has made a promise to protect Ino at all costs. There might be a prophecy or some sort of dark force threatening the village and Ino is specifically targeted. Naruto, knowing the importance of her life for the village or for some other deeper reason, gives up his own life to ensure her safety. This could lead to a great deal of character development for the other characters as they deal with Naruto's sacrifice.
One possible plot could be that Naruto sacrifices himself to save Boruto from a powerful new enemy. Maybe an ancient evil resurfaces that is too strong for Boruto to handle alone, and Naruto steps in, using up all his chakra and ultimately dying in the process.
An unexpected character could be responsible. Maybe a character that Harry thought was harmless but was secretly envious of him. This character could have used some form of emotional manipulation, constantly undermining Harry's confidence and self - worth until his heart was broken and he died.
One possible scenario could be that Harry loses the love of his life. Maybe she was killed in a battle against dark forces, and he is unable to move on from that grief. His broken heart then leads to his own death as he slowly loses the will to live, consumed by his sorrow and memories of her.
One possible plot could be that Naruto dies heroically in a final battle against a powerful enemy. As a ghost, he discovers that there are unresolved issues in the Hidden Leaf Village. He then tries to communicate with his friends like Sasuke and Sakura, but they can't see him at first. Eventually, he finds a way to make his presence known and helps them solve a mystery that was related to an old enemy's plot that was still endangering the village even after his death.
One possible plot could be that an ancient evil is awakened and sets its sights on destroying Konoha. Naruto, as the Hokage or a respected ninja, rallies all his friends and allies at first. However, as the battle progresses, he realizes that the only way to truly stop this evil is by giving his own life. So he makes the ultimate sacrifice, using his chakra to seal the evil away forever, thus protecting Konoha at the cost of his life.
One possible plot could be that Naruto arrives in heaven and meets all the past Hokages. He has deep conversations with them about his life, his struggles, and how he changed the ninja world. Maybe he also gets to see his parents again and they have an emotional reunion, sharing all the things they missed out on while he was growing up.
Perhaps the village is invaded by a horde of powerful demons. Naruto, after a long and grueling battle, realizes that the only way to stop them is to seal them all away, like the way the Fourth Hokage sealed the Nine - Tails. But this sealing jutsu demands the life of the caster. Without hesitation, Naruto performs the jutsu, sacrificing himself to save the village he loves so much.
One possible plot could be that Naruto dies heroically in a great battle against a powerful enemy. Then, he is reborn into a world that is very different from his own. He might retain some of his past memories, which help him to quickly gain strength and knowledge in this new world. He could use his new - found situation to prevent the same tragedies that occurred in his previous life.