In a 'naruto after the war harem fanfiction', the plot might involve Naruto being the hero of the war and now many female ninjas are attracted to him. Some of them could be from different villages. They start to compete for his attention while Naruto is focused on peace - building, creating a complex web of relationships.
Naruto is the main character, of course. Hinata is very common as she is Naruto's wife in the original series. But other characters that often show up are Temari. Since she has interacted with Naruto on various occasions and has a strong personality. Another one could be Kurenai, who, although older, could bring a different kind of maturity and wisdom to the harem dynamic in the fanfiction.
One possible plotline could be Naruto having to adjust to his new relationships within the harem. For example, dealing with the different personalities and needs of each girl. Maybe one girl is from a different village and there are cultural clashes that Naruto has to mediate. Another could be about how Naruto uses his newfound family - his harem - to help him rebuild the village in a more harmonious way.
Sakura could be one of the main female characters. She has always had a connection with Naruto, and after the war, her feelings might have changed or deepened. Hinata is also a likely candidate as she has long - admired Naruto. And perhaps some new characters from other villages could be introduced as well.
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 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.
He could meet an old enemy who has also changed after the war. This enemy might now be in a similar situation of being alone and they could form an unexpected bond as they understand each other's isolation.
Naruto could encounter wandering ninjas who have their own stories and reasons for not being part of a village. Some might be outcasts due to their beliefs or powers, and Naruto's interaction with them could lead to either conflict or new friendships. He may also come across some of the shinobi who fought on the opposite side during the war but now have different outlooks on life. Through these meetings, Naruto gets a broader perspective on the ninja world.