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.
Maybe in this fanfiction, Naruto is alone after the war because the villagers still don't fully trust him despite his heroic deeds. He then embarks on a journey to find a new purpose, away from the village that he once fought so hard to protect. Along the way, he discovers new powers within himself and also meets some unlikely allies who help him come to terms with his situation.
In this fanfiction, Harry could be alone because he feels estranged from his friends who have moved on with their lives in different ways. So he takes on a solitary mission to protect the wizarding world in the shadows. He becomes a sort of unseen guardian, constantly thwarting small threats that could potentially grow into big problems, all while dealing with his own loneliness and the memories of the war.
One possible storyline could be Percy struggling with PTSD. He might have nightmares about the war and find it hard to reintegrate into normal life. For example, he could avoid social situations because the noise and crowds remind him of the chaos of the battlefield.
One possible storyline could be Harry struggling with PTSD from the war. He might isolate himself in Grimmauld Place, haunted by the memories of those he lost. Another could be him trying to find a new purpose in life, perhaps traveling the wizarding world alone to discover forgotten magic. Or he could be dealing with the public's perception of him as the 'Chosen One' and feeling the pressure to always be strong and heroic.
A potential character could be a reclusive old wizard who lives on the outskirts of the wizarding world. This wizard might have his own reasons for living in seclusion, but when he sees Harry's loneliness, he takes him under his wing. Another character could be a young, orphaned witch who is also trying to find her place in the post - war wizarding world and she crosses paths with Harry.
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.
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.