In some cases, he could become a puppet for the dark side. Without his soul, he has no moral compass, so he could be easily manipulated by the forces of evil to do their bidding. His friends would then be in a race against time to restore his soul before he causes too much harm.
Maybe he was hit by a powerful dark magic spell specifically designed to target the soul. Such spells in the Harry Potter universe can be extremely dangerous.
One consequence could be that his relationships change. His friends might not recognize the new 'soul - changed' Harry at first, leading to some initial confusion and distance in their friendships.
In some fanfiction concepts, he might change his soul through powerful magic spells. Maybe he discovers an ancient spell in a hidden book in the Room of Requirement that has the power to transform his very essence, his soul.
There are indeed 'Harry Potter sold his soul' fanfictions out there. It's a very interesting, if somewhat disturbing, concept. In some of these fanfictions, the author might use it as a way to explore different aspects of the wizarding world that were not fully explored in the original books. For example, they could show how the dark magic associated with selling one's soul interacts with the existing magical laws and institutions in the wizarding world. It could also be a way to look at Harry's character from a completely different perspective, seeing how his personality might be altered by such a huge sacrifice and the subsequent influence of dark forces.
In this kind of fanfiction, Harry might feel extremely lost and depressed. He has been defined by his magic for so long, and without it, he'd have to re - evaluate his identity. Maybe he'd start to rely more on his friends, like Ron and Hermione, in different ways. For example, instead of using magic to solve problems, he'd have to use his wits and their combined non - magical skills.
One common theme is the exploration of darkness. Since selling one's soul is a very dark act, fanfictions often focus on how Harry's world becomes filled with more sinister elements. Another theme could be the loss of self. Harry might struggle with losing his identity as he is influenced by the dark forces that now have a claim on his soul.
The plot could progress with Harry using magic to compensate for his lack of glasses. For example, he could use a spell to make his eyesight better. Or, his friends could use their skills to help him. Hermione might use a spell to locate his glasses in the chaos of the brawl.
In such a fanfiction, Harry might be at a serious disadvantage. Without his glasses, he'd struggle to see clearly. His opponents could easily take advantage of his impaired vision. He might have to rely more on his other senses, like his hearing or his magical intuition. Maybe he'd cast a spell to enhance his vision temporarily, but it could backfire or be disrupted during the brawl.
One possible reason could be the overwhelming burden of his destiny. Constantly facing dark forces and the pressure of saving the wizarding world might have worn him down. Another reason could be the loss of his loved ones. For example, if in the fanfiction Hermione or Ron were killed, that could shatter his spirit.