Maybe it's an overzealous Hogwarts professor who suspects Harry of being a dark wizard due to his strange appearance in the past. This professor could constantly try to thwart Harry's attempts to find a way back to his time and also try to drive a wedge between him and Snape.
Well, one possibility is that Harry could accidentally trigger a time - turning spell while in Hogwarts. Once stuck in the past, he might have to deal with a younger Severus Snape. Maybe he tries to change some events related to Snape's past to improve their relationship in the future.
There's a chance that he'll meet some sort of magical creature that has made the cupboard its home. This could be a friendly creature that helps him get out or a more mischievous one that makes his situation even more difficult. He'll have to use his wits and magic skills to deal with it.
He could find himself in a really tough situation. For example, if he's in the middle of a feud between two magical factions in the fanfiction. He would have to use his wits and skills to try and find a way out or a solution that satisfies both sides. He might also start to question his own beliefs and loyalties as a result of being in this middle position.
Dark themes are prominent. This can include things like dark magic, power struggles within the wizarding world that directly affect their relationship. In these fanfictions, the characters may be forced to make difficult choices that go against their normal moral compasses. And often, the settings are gloomier, like Hogwarts during a particularly bad time or in some desolate magical place.
One popular 'dark harry snarry fanfiction' is 'The Binding'. It delves deep into the complex relationship between Harry and Snape in a rather dark and intense setting. The story plays with power dynamics and the characters' inner turmoil.
Maybe it could be Godric Gryffindor. Since Harry is in Gryffindor house, Gryffindor himself might take it upon himself to train Harry, passing on his courage - based magic and sword - fighting skills.
Rather than relying solely on finding an existing fan fiction, you could also consider writing your own. If you're a fan of the Snarry pairing and are interested in exploring the idea of Harry getting sick from abuse in this relationship, you can start with creating an outline. Think about the nature of the abuse, how it makes Harry sick, and how Severus Snape reacts to it. Then build the story from there.
In the fanfiction, Pooh could get stuck in a time - loop type of situation. He keeps repeating the same actions over and over again, like climbing a tree to get honey but always ending up back at the bottom. This would be a really interesting and puzzling situation for Pooh, and he would have to figure out how to break the loop with the help of his friends or by learning something new about himself.
One great 'Harry Potter' fanfiction with a Metamorphmagus in a Snarry pairing is 'Metamorphosis of the Heart'. In this fic, the concept of the Metamorphmagus adds an interesting layer to the relationship between Snape and Harry. It allows for unique interactions and character developments as the Metamorphmagus ability can be used to show emotions and changes in a very physical way within the story.