One possible plotline could be Harry feeling extremely isolated and trying to find new friends outside of his usual circle. Maybe he starts to focus more on his studies and discovers new magical abilities on his own.
One consequence could be a breakdown in their friendship. His friends might feel hurt and start to distance themselves from Harry. For example, Hermione, who is very loyal, might be deeply offended if Harry constantly doubts her.
In some fanfiction, the author might create a plot where Harry has been betrayed before. So, he has trust issues even with his friends. For example, if he was tricked by someone who seemed like a friend in the past, it could make him more cautious and less trusting in new situations with his real friends.
The Ministry of Magic could also be the main antagonist in this case. If the Ministry is corrupted or under the influence of dark forces, they could manipulate Harry's friends. They might convince his friends that Harry is a danger to the wizarding world order, and his friends, being naive or misinformed, end up betraying him.
Well, usually in such fanfictions, Harry might face a lot of emotional turmoil and have to find his way alone to overcome the betrayal and the challenges that come with it.
The plot might involve a curse. Naruto could be cursed by an ancient power. As a result, his friends' memories of him start to fade away. He might notice this when they stop responding to him as they used to. Naruto would be heartbroken but determined. He could search for the origin of the curse, facing many challenges along the way. And in the process, he might discover some long - lost secrets about his own family or the ninja world that are related to this curse.
The length of the fanfiction can also play a role. Longer and more complex fanfictions that require a big time investment to read might have been forgotten. People might have started them but not finished, and as new, shorter, and more accessible fanfictions became available, these longer ones got pushed aside. Additionally, if the writing style was very different from what was popular at the time, it could have been overlooked. For example, if it was written in a very experimental or archaic style, it might not have gained a wide readership and thus became forgotten.
Later, he could become depressed. Harry has always valued his friends highly, so losing them would be a huge blow. He might isolate himself in his dormitory, not participating in school activities as much. He would probably be constantly thinking about what went wrong and how to fix it.
Harry's acceptance of his destiny would make his relationships with his friends more complex. On one hand, they would be proud of him. But on the other, they would have to adapt to his new role. His friends would need to find their own places in his journey. Hermione might focus more on researching ways to help Harry fulfill his destiny, while Ron could become more of a protector, guarding Harry from distractions. This evolution in their relationships would be a crucial part of the fanfiction's story.