Well, often in these fanfictions, Harry goes to Knockturn Alley out of curiosity or in search of something specific. There, he may run into characters who have ties to the Dark Arts. For example, he could meet a wizard who is an expert in dark potions and has to use his wits to get out of a tricky situation without getting involved in any illegal or dangerous activities.
In certain fanfiction stories, when Harry visits Knockturn Alley, he might stumble upon a secret society. This society could be planning something that endangers the wizarding world. Harry, being the hero he is, would then try to gather information about their plans. He might also find ancient artifacts that have a connection to Voldemort's past, which could lead to a whole new adventure as he tries to figure out how to use or destroy these artifacts to prevent more harm to the wizarding community.
He might meet some shady characters like Borgin and Burkes. They deal in all sorts of strange and often dark items.
In certain fanfiction stories, when Harry goes to Gringotts, he could be on a quest to retrieve a powerful magical artifact. Along the way, he may meet new characters, either friendly or hostile. He might have to fight his way through various security measures, which could include things like enchanted doors and traps. And his visit to Gringotts could be a turning point in the story, setting off a chain of events that will change his life and the wizarding world around him.
In many Harry Potter fanfictions about Knockturn Alley, there are some really interesting shops. One might be a store that focuses on selling items for the Dark Arts, like special robes that have been enchanted with dark magic for concealment or intimidation. There could also be a shop that offers services to repair or modify dark magical objects. Additionally, there may be a place that sells strange and otherworldly substances that are used in the most complex and dangerous dark spells, which are often sought after by dark wizards or witches who are looking to expand their powers in the shadowy side of magic.
In fanfiction, Harry Potter going to France could lead to an adventure where he gets involved in a mystery related to a powerful French magical artifact. He'd have to team up with local wizards to solve it and in the process learn about the rich history of French magic.
He could meet a famous wizard author who is signing books in Diagon Alley. This wizard could give Harry some valuable advice about his future.
He could meet a young and brilliant wizard inventor. This inventor is working on some newfangled magical gadgets and shows Harry some prototypes, like a self - flying broomstick that can change its shape according to the rider's mood.
He could have a very spooky experience. For example, in a shop full of cursed objects, he might feel an oppressive atmosphere and strange whispers. The items might seem to have a life of their own and try to draw him in.
Often, we see characters like Draco Malfoy in Knockturn Alley fanfiction. Given his family's connection to the dark arts, it's easy to imagine him frequenting the place. Then there are the shopkeepers of the various dark magic shops, like Borgin and Burkes. And sometimes, even Harry Potter himself might be written into a story set in Knockturn Alley, perhaps on a mission to uncover some dark plot.
The environment itself in Knockturn Alley is a danger. It's dark, dingy, and full of shadows where someone could easily lurk and attack him. There could be traps set by dark wizards for unsuspecting victims, and Harry, being in hiding, might not be as aware of his surroundings as he should be. Moreover, some of the potions and spells being sold there illegally could be used against him if discovered.
One interesting storyline could be a young wizard getting lost in Knockturn Alley and having to find their way out while facing various dark magic items and shady characters. Another could be about a character uncovering a secret illegal potion - making operation in the alley. And there might be a story where a wizard tries to reform some of the dark wizards in Knockturn Alley.