There might be a new dark wizard or witch that we don't know in the main series. This new character could be extremely powerful and target Harry specifically. They might have a vendetta against Harry's family or just want to take over the wizarding world by first destroying Harry, who is seen as a symbol of hope, and thus make him lose everything.
Perhaps in this fanfiction, Harry loses his family's legacy. For example, something might happen to make people believe that his parents were not the heroes they were thought to be. This would not only shatter his own self - image but also make him lose the respect and support he had from the wizarding community. It would be a huge blow and make him feel like he's lost everything.
It could be Voldemort. Even if he's defeated, his actions might have left such deep scars on Harry. For example, all the people he killed and the destruction he caused. Harry might feel responsible for not being able to prevent more deaths and this guilt could make him lose the will to live.
Perhaps it's an old enemy that was thought to be defeated. This enemy might have been biding their time and finally finds a way to get revenge on Harry by driving him mad. It could be a Death Eater who was overlooked or a creature from the Forbidden Forest with a grudge.
It could be a hard and challenging one. He might have to start from scratch, face lots of difficulties, and find the strength to rebuild his life.
It could be about a fan - created story where the character Harry has ownership over everything. Maybe it's in a fictional world where he has amassed great wealth, power, or all sorts of possessions.
There are various possibilities. He might have a sudden stroke of luck and get back on his feet. Another option is that he leans on the support of friends and family to slowly regain what he's lost. Sometimes, though, people in such a situation struggle for a long time before seeing any improvement.
One common theme is isolation. When Harry loses everything, he often finds himself alone, cut off from the friends and the world he once knew. Another theme is despair. Losing everything makes him question his existence and purpose. For example, in many such fics, he might wonder why he should keep fighting when he has lost so much.
In fanfiction, 'Harry Potter remembers everything' can be a really interesting concept. It might mean that Harry has a perfect recall of all his experiences, including the traumas. This could lead to him being more strategic in dealing with Voldemort and his followers. For example, he could remember every detail of a Death Eater's spellcasting pattern and use that knowledge to his advantage in battles.
It could introduce new original characters that are central to the story. Maybe a young witch or wizard from a foreign wizarding school who comes to Hogwarts and has a major impact on the events in the fanfic. This new character could bring different magical abilities and cultural elements from their own school into the Hogwarts environment, causing all kinds of changes.
They had to face many challenges and start from scratch. Maybe they made some bad business decisions or faced unexpected disasters.