If we think about it, the plot could start with Harry suspecting Mary Snape of being involved with some dark plot. He might be forced to take matters into his own hands and force her to cooperate. Maybe Mary Snape has some knowledge about a new dark wizard or a hidden spell that could cause chaos. Harry, in his desperation to protect the wizarding world, resorts to using force. As the story progresses, he could discover that Mary Snape was actually innocent all along, and then has to deal with the consequences of his actions, like mending the relationship with Mary and facing the judgment of his friends and the wizarding community.
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.
In a 'Harry forced Mary Snape' fanfiction, the plot could be really complex. Maybe Mary Snape is a half - blood who has been ostracized by both the pure - bloods and some of the half - bloods in the wizarding world. Harry, believing she has some connection to a new threat, forces her to come with him. Along the way, they start to understand each other better. Mary Snape might have her own reasons for her actions, and Harry realizes that his initial judgment was wrong. This could lead to a story of redemption, where Harry has to make amends for forcing her, and they both end up working together against a common enemy that is actually a greater threat than Harry initially thought. It could also explore themes of prejudice, forgiveness, and the gray areas between right and wrong in the wizarding world.
Finding 'Harry forced Mary Snape' fanfiction could be a bit of a challenge. Since it's not a mainstream concept within the Harry Potter fan community, it may not be as easy to come across as more popular fanfiction themes. Some fanfiction authors might be more inclined to write about the relationships and events that are more closely related to the original books, like the relationship between Harry and his friends or the battles against Voldemort. But that doesn't mean there isn't a story out there with this particular concept. You could also check some niche Harry Potter fanfiction communities or forums where fans might discuss or share such unique stories.
In this fanfiction, it could also be that Harry's Gryffindor nature starts to rub off on Snape in some ways. They might have arguments about how things should be done, but through these interactions, Snape might start to see the value in Harry's way of thinking. And Harry, on the other hand, might learn some of Snape's more cautious and calculating ways, which could help him in his future battles.
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.
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 reason could be a misfired spell by another character. For example, if a young wizard was experimenting with a transportation spell and it backfired, it could accidentally force Harry to be attached to Snape in this 'ride' situation. Another reason might be a plot by a dark wizard who wants to see chaos at Hogwarts. They figure that forcing Harry and Snape together in this strange way would disrupt the normal order of things.
Maybe some old pure - blood families who want to regain power. They view Harry as a symbol of the new order that they oppose, and Snape as a tool they can use. By forcing this marriage, they hope to control both of them and use their combined influence for their own dark purposes.
Well, Snape could force Hermione to participate in a wizarding duel against her will. In fanfiction, this could be a way for Snape to test her skills or perhaps punish her for something he believes she has done wrong. Hermione would be in a tough situation as she is a rule - abiding student, but being forced into a duel would make her have to think on her feet and use her magic in new and unexpected ways.
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.
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.