Well, he might start to explore a new career path in the non - magic world. For example, he could become a writer and write about his past magical experiences from a new perspective.
If Harry gives up magic, he'd likely move into the Muggle world. This would open up new storylines. He could start a new career in the Muggle world, perhaps in education or some other field. His friends from the wizarding world might visit him secretly, and there could be a whole new set of adventures based on the clash of the two worlds, like how he hides his magical past from Muggles and deals with the occasional magical emergency that still manages to find him.
It could be that he got too busy with other aspects of his life. For example, he might have started a new job or hobby that took up all his time, leaving no room for writing or being involved in magic fanfiction.
One possible reason could be that he has seen too much destruction and chaos caused by magic. He might feel that it brings more harm than good, so he decides to give it up.
One possible reason could be that J.K. Rowling, the author of Harry Potter, has strict control over the franchise and might have made some regulations that made it difficult for him to continue. Another reason could be that he simply lost interest in writing fanfiction about it. Maybe he wanted to focus on creating his own original stories instead.
One possible reason could be that Ginny realizes Harry is too preoccupied with his heroic duties and she feels neglected. In fanfiction, this is a common trope where the female character desires more attention in a relationship.
One possible reason could be a series of misunderstandings. Maybe his family made decisions that he felt were against his principles or dreams, and after repeated attempts to reconcile the differences, he just lost hope. For example, if his family was against his ninja way, which is such a core part of his identity.
One reason could be the constant setbacks he faces. He might feel that no matter how hard he tries, he can't achieve his goal. For example, if he fails too many times in battles against strong opponents, he could lose hope. Another reason might be the loss of important people in his life. If someone like Iruka or Kakashi, who have been his pillars of support, were to die, it could shatter his dreams as they are so closely tied to his will to succeed for their sake.
The story could take a dark turn. Without his heart, Naruto might become a soulless being, and his friends would be devastated. They might try to find a way to get his heart back. Maybe they search for a powerful sage or some ancient magic that could reverse what has been done.