Perhaps a new, unknown force. With Naruto gone, a previously hidden group with a vendetta against the ninja villages could emerge. They might blame the villages for the destruction caused during the war and see Naruto's death as the perfect time to strike, when the villages are still recovering and vulnerable.
One possible plot development could be that the Hidden Villages struggle to maintain peace without Naruto's influence. His death might lead to power vacuums and political unrest. For example, some villages may try to take advantage of Konoha's weakened state. Another aspect could be how his friends and family cope. His wife, Hinata, might have to raise their children alone, and his friends like Sasuke would feel a great sense of loss and perhaps guilt for not being able to protect him. The ninja world as a whole could also lose its hope symbol, and new, darker forces might emerge, thinking that without Naruto, they can carry out their evil plans more easily.
Yes, there are some. Some fanfictions explore this dark concept in interesting ways. They might show how his death impacts the shinobi world, like the grief of his friends and the power vacuum it creates.
One of the most heartbreaking scenes could be when his friends find out about Naruto's death. For example, Sakura, who has been with Naruto through so much, would be completely shattered. Her reaction of disbelief and then overwhelming grief would be really sad. Another scene might be when the whole village of Konoha hears the news. The silence that would fall over the village, with people just stopping in their tracks, would be a very poignant moment.
If Naruto dies in the Fourth Shinobi War in fanfiction, it would have a huge impact. For one, the morale of the Allied Shinobi Forces would plummet. Naruto was the symbol of hope. Without him, many might lose the will to fight. His friends like Sasuke would be deeply affected. Sasuke's whole journey of redemption was somewhat tied to Naruto. His death could also lead to a power vacuum. Nations that were relying on Naruto's strength and his idea of peace might fall into chaos as they scramble for power or new forms of protection.
In some fanfictions, Naruto becomes more jaded after the war. He has seen so much death and destruction that he loses some of his initial idealism. He might be more cautious in his actions and less quick to trust.
In some fanfictions, Naruto and Hinata start a family right away. They enjoy a peaceful life in Konoha, with Naruto being a great Hokage and Hinata supporting him at home and also taking part in some village activities.
One common theme is rebuilding. After the war, the villages, especially Konoha, need to be rebuilt both physically and socially. Another theme is reconciliation, like Sasuke coming back to Konoha and being accepted again. Healing is also a theme, as the characters have to deal with the mental and physical scars of the war.
One common theme is sacrifice. Naruto gives up his life to ensure victory in the war. Another is the impact on his friends and family. They have to deal with his loss.
One possible reason could be to protect his family. After the war, Naruto might have made many enemies. By faking his death, he could draw the attention of these enemies away from his family, especially his wife and children. His enemies would think he's no longer a threat and stop targeting his loved ones.