The plot might start with a magical experiment gone wrong. Harry ends up with a special mark or power that makes him Snape's master. As the story progresses, Harry uses this power to make Snape teach him advanced potions and dark arts defenses. Snape resents it but has no choice. Meanwhile, Voldemort senses this strange new power dynamic and tries to use it to his advantage. This forces Harry and Snape to work together in ways they never thought possible to outwit Voldemort while also dealing with their own difficult relationship.
Sure, the Harry Potter fandom is huge and full of creativity. Writers might come up with a story where Harry is Snape's master for various reasons. It could be due to a prophecy or a magical bond that no one expected. In this kind of fanfiction, we might see Snape having to follow Harry's orders, which would be a complete reversal of their normal relationship in the original series. This could lead to some very intense and dramatic storylines.
Maybe in the fanfiction, Voldemort devises a plan where Snape forgets Harry to disrupt the Order of the Phoenix's plans. Without Snape's knowledge of Harry and his role in the prophecy, things start to go awry. Harry has to find a way to make Snape remember, perhaps by going on a quest to find a magical relic that can restore memories. Hermione and Ron would be by his side, of course, providing their usual support and knowledge.
One possible plot twist could be that Snape has a hidden agenda all along. Maybe he's been using Harry as a pawn in a bigger scheme related to the wizarding world's power struggle. Another could be that Harry discovers a long - lost family connection to Snape that changes their dynamic completely.
One possible plot development could be that Harry initially struggles with having Snape under his 'command' in some way. Since they had a very strained relationship in the original series, Harry might be hesitant to trust Snape even if Dumbledore has given him this new role. He could start by trying to figure out Snape's true intentions.
The plot could be that Dumbledore assigns Harry to clean Snape's home as a way to build a bond between them. Harry initially resents it but as he cleans, he finds a diary that reveals Snape's unrequited love for Lily. This discovery makes Harry see Snape in a new light and he decides to try and befriend him. He starts to clean with more care, and when Snape returns, he is initially angry but then realizes Harry has a newfound respect for him.
In a 'Harry sold rescued Snape' fanfiction, the plot could start with Harry learning that Snape is held captive by a group of dark wizards. Harry realizes that he needs certain magical supplies or potions to break Snape free. Since he doesn't have the funds to buy these items, he decides to sell a rare spell book that he found in the Room of Requirement. This spell book has some unique spells that could potentially be used against the dark wizards. But once he sells it, he discovers that the buyer has some ulterior motives and is somehow connected to the group holding Snape. Now Harry has to not only get the spell book back but also figure out how to use it to rescue Snape while dealing with the new threats that have emerged.
Well, it's hard to say exactly without reading an actual piece of such fanfiction. But perhaps it could involve Harry being in some sort of difficult situation where he has to force Mary Snape (assuming she's an original character) to do something for the greater good, like revealing a secret that could help in the fight against dark forces. Or maybe it's a misinterpretation situation where Harry thinks Mary Snape is hiding something dangerous and he has to use force to get to the truth.
One possible plot could be that Snape, despite his initial resentment towards James Potter (Harry's father), sees the vulnerability in little Harry. He decides to take him in out of a sense of duty to Lily (Harry's mother). Snape then has to adjust his life to care for a child. He teaches Harry potions at an early age and tries to protect him from the darker elements in the wizarding world. Harry grows up with a different perspective on Snape than in the original story.
One possible plot could be that Harry starts to notice Snape's hidden kindnesses towards him. Maybe he begins to see past Snape's cold exterior during potions class or in their interactions in the hallways. This newfound perspective makes Harry develop a crush. As he tries to get closer to Snape, it causes chaos and confusion among the other characters at Hogwarts.
One possible plot could be that Harry discovers he has a hidden connection to Sebastian Snape, perhaps a long - lost family relation. This revelation could lead him to question his identity and his place in the wizarding world.