Perhaps his uncle has a very intimidating appearance or a loud voice that scares Harry. In the fanfiction, it could be that his uncle has a certain look in his eyes when he's angry that makes Harry tremble with fear.
It could potentially make Harry more empathetic towards others who are in fearful situations. Since he has experienced such intense fear himself, he may be more likely to understand and help those who are also afraid. In the fanfiction, this could be shown through his interactions with other characters who are facing their own fears.
Uncle Wu Zhu was afraid of fire. The Qing Emperor and the other characters all knew that Wu Zhu's weakness was his fear of fire. Wu Zhu was a robot. His body was made of a special alloy. Although it looked like human flesh and blood, it was essentially metal. Many years ago, the Qing Emperor had seen the Temple emissary melt into a strange thing by fire on the wasteland. This made him believe that fire was Wu Zhu's weakness. As a result, the Qing Emperor had no confidence in Wu Zhu and knew that he could not defeat him.
Yes, there are some. Some fanfictions explore the darker side of Harry's life with his uncle in great detail. They often delve into the emotional and psychological abuse he might have endured. However, the quality can vary greatly from one to another.
Dumbledore could step in. As the most powerful wizard in the wizarding world, he has a deep connection with Harry. He would likely use his vast network of contacts and his own magic to find Harry. Dumbledore would also be concerned about the implications of Harry being sold and would want to protect him from any potential harm.
In the fanfiction, Harry might be afraid of doctors because he has seen other people have terrible experiences with them. He could have witnessed a friend go through a lot of pain during a medical procedure, and that made him fear doctors.
One possibility could be that Harry, out of a sense of duty or some form of magical coercion, decides to follow his uncle's every order in order to protect his aunt and Dudley from a new magical threat. He might suppress his own desires and instincts to keep them safe.
One possible plot could be that Harry is sold to a dark wizard. The wizard might want to use Harry for his own dark purposes, perhaps related to the power he holds as 'The Boy Who Lived'. Harry then has to find a way to escape and seek help from his friends at Hogwarts.
One possible reason could be past trauma. Maybe he had a bad experience in his childhood, like being bullied or physically hurt by someone. Another reason might be related to his magical abilities. He could be afraid that being touched might somehow disrupt his magic or cause an unexpected magical reaction.
On the other hand, it could be positive in a way. Maybe through submitting, Harry learns more about his uncle's insecurities and past, which could lead to a new understanding and a form of reconciliation between them.