Perhaps he meets a mysterious character who also has an interest in the dark side but in a more dangerous way. Harry initially thinks they can be friends and learn from each other, but as time goes on, he realizes the true nature of this person's intentions. This could lead to a conflict where Harry has to decide whether to expose the person or keep quiet to protect his own secret.
As he delves deeper into his hidden interest, Harry might start to notice changes in his own magic. It could become more unpredictable or powerful in a way that he can't fully control. His friends start to suspect that something is wrong, and he has to constantly come up with excuses to avoid their questions. This could lead to a rift in his friendships and put him in a very difficult position within the wizarding community.
The plot could develop with Harry secretly studying dark magic texts in the restricted section of the library. He might start to have strange dreams or experiences related to the dark magic he's exploring, and he has to be very careful not to get caught by Filch or the teachers.
He might hide it because he doesn't fully understand his own fascination. Harry has always been on the side of good, but perhaps there are elements of the dark side that seem alluring in a strange way. He could be afraid that exploring these feelings would lead him down a dangerous path, so he keeps it a secret.
In the 'Harry Potter Hides He Is Dark' fanfic, Harry could be hiding his dark side by carefully choosing his words. He wouldn't let slip any thoughts or opinions that might hint at his darker tendencies. He might also avoid certain places or people that could trigger his dark side, like staying away from dark magic artifacts or those who practice dark magic openly. And, of course, he could be using Occlumency not just to keep his thoughts private from Voldemort but also to hide his own dark nature from others.
The revelation of Harry being 'dark' would cause chaos in the wizarding world. People who once idolized him would feel betrayed. The Order of the Phoenix might question their support for him. Enemies could use this as an opportunity to turn the public against Harry. His relationship with Ginny would also be severely strained. She fell in love with the 'good' Harry, and this new side of him could be too much for her to handle. Also, it could potentially put Neville or Luna in danger if they continued to support Harry, as they might be seen as associated with his 'darkness'.
Well, it could be that Harry Potter discovers he has a connection to Sparda. Maybe he has some latent power that is related to Sparda's abilities. He then has to learn to control this new power while still dealing with the threats in his own wizarding world. And perhaps he uses his new - found powers to fight off dark wizards in a more badass way.
Maybe in this fanfic, Harry discovers a prophecy that is related to her gender. To avoid fulfilling a part of it that could lead to disaster, she hides her gender. In the process, she could form new alliances with female students who are more in tune with her new 'female identity'. She might also face challenges in classes like Defense Against the Dark Arts because the teaching methods are sometimes more tailored to male students in the original Hogwarts setup.
The plot would take a very dark turn. Harry would no longer be the hero we know. He might start working against the Order of the Phoenix. His actions would be unpredictable and often evil - like attacking Hogwarts instead of protecting it.
The plot could start with Harry being sorted into Ravenclaw, which surprises everyone. Then he begins to discover the unique aspects of Ravenclaw house. He might get involved in a mystery related to a lost Ravenclaw heirloom. As he tries to solve it, he meets other Ravenclaw students with different areas of expertise, and together they use their collective intelligence to follow the clues.
He might go to America. The American wizarding world is different from the British one. There are different magical creatures and a different magical culture. Harry could go there to start anew, maybe help out at Ilvermorny or explore the magical wilderness of the States.
He may be confused at first. Fanfics can be very different from his real experiences. He might wonder why people write such things about him and his world.
At first, it would be really rocky. Harry would be suspicious of Snape and Snape would be his usual grumpy self. But over time, as they get to know each other better, they might start to respect each other. Harry could learn about Snape's past and his complex relationship with his mother, which might soften his view of Snape.