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.
It might lead to a decrease in the overall quantity of Harry Potter fanfiction. If he was a well - known or prolific writer in the community, his absence could be felt. Other writers might be inspired to fill the gap he left, though.
Perhaps in the fanfiction, Harry gives up because he becomes disillusioned with the wizarding world. He may see all the corruption and prejudice within it and decides that Voldemort's way of ruling might not be much worse. This could lead to a story where Harry actually joins Voldemort's side, not as a slave but as an 'advisor' of sorts, trying to change Voldemort from the inside. However, this is a very dangerous and morally ambiguous path for Harry to take.
Maybe in the fanfiction, Harry discovers some dark secrets about his past that make him question his whole existence and purpose. Like if he finds out that his parents' sacrifice was part of some bigger, more sinister plan, he could become so disillusioned that he gives up on life. He might think that everything he has been through was a lie and there's no point in going on.
It might imply that within the context of a Goblet of Fire - themed fanfiction, Harry Potter reaches a point where he can no longer continue. He could be tired of the events, or perhaps he realizes that the path he was on in the fanfiction was wrong. For example, if the fanfiction had him in a dangerous situation in the Triwizard Tournament that was getting out of hand, he might choose to give up to protect himself or others.
Voldemort and Harry's relationship would be very strange. Voldemort might not trust Harry at first, even though he has given up. He would constantly be on guard, thinking it's a trick. And Harry, on the other hand, would have to navigate living in the Death Eaters' world. The other Death Eaters would also be suspicious of Harry, some might even try to kill him to prove their loyalty to Voldemort.
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.
After Ginny gives up on Harry, Harry himself could be forced to re - evaluate his life. He might realize how much he has taken Ginny for granted. This could lead to a period of self - reflection for Harry, and he may try to win Ginny back, which would create a whole new set of plot developments.
Hermione could be a main character. She might be the one trying to pull Harry out of his despair. Or she could be in a similar state herself, having witnessed Harry's downward spiral, which adds to the overall gloom of the story.