Maybe it's a political move within the wizarding world. The Ministry of Magic might force this marriage to unite two powerful magical forces. Harry would then have to deal with his feelings for his friends like Hermione and Ron, who would be shocked. Snape, on the other hand, would have to adjust his view of Harry from a student to a partner. They could start to find common ground in their love for Hogwarts and magic in general.
One possible plotline could be Harry trying to find a way to break the forced marriage bond while secretly gathering allies to fight Voldemort.
Perhaps there could be a magical object or spell involved that is causing this forced marriage. Harry and Snape might have to work together to find and break this spell. Along the way, they could learn more about each other's pasts and secrets, which could change their relationship from one of enmity to something more complex.
One possible plotline could be Harry initially resisting the marriage due to feeling forced into it, but as he spends more time with Ginny, he discovers new sides of her and gradually falls in love. For example, they might have to work together to face some new dark magic threat in the wizarding world, and through that shared experience, their relationship deepens.
One possible plotline could be that it's a political arrangement. Maybe there's some sort of wizarding world politics where uniting the Potter and Greengrass families is seen as beneficial for stability. Harry might initially resist but then gradually find out that Daphne has her own charm and they start to develop a relationship based on mutual understanding. For example, they could bond over their shared experiences in the wizarding world, like dealing with pure - blood prejudices.
One common theme is power struggle. Since Snape has his own power and influence in the wizarding world, and Harry is the 'Chosen One', a forced marriage between them could be a way for different factions to gain control. Another theme could be identity crisis. Harry would surely struggle with his identity as a Gryffindor hero being married to Snape, a former Death Eater and Slytherin.
One possible plotline could be a magical misadventure where Harry and Snape are forced into an awkward situation due to a spell gone wrong. For example, a spell binds them in a strange position as a prank by a mischievous magical creature. Another could be a mind - controlling potion that makes Harry act in this odd way towards Snape, but it's all part of a mystery to uncover who is behind it. And perhaps there could be a situation where they are in a magically - created obstacle course and this strange 'forced to straddle' situation is a part of one of the challenges they must overcome together to save Hogwarts from a new threat.
Yes, there are some. You can start by looking on fanfiction platforms like Archive of Our Own. However, the idea of Harry being forced to marry Snape is quite an unusual and often dark or complex concept in fanfiction. Some stories might explore this in the context of a magically - bound contract or some sort of political intrigue within the wizarding world.
In some 'Snape slave to Harry fanfiction', the plot might start with a prophecy or a magical bargain. Maybe a seer predicted that only with Snape as Harry's servant could a great evil be defeated. So, Snape, bound by this fate, has to follow Harry's commands. Along the way, they face many challenges together. Harry has to deal with the power imbalance and Snape has to struggle with his pride. Their relationship evolves from one of forced servitude to a more complex bond of respect and cooperation as they fight against the forces of darkness.
One possible plotline could be that Snape accidentally discovers Harry is extremely ticklish and uses it as a form of mild punishment when Harry misbehaves in Potions class. For instance, if Harry forgets an ingredient or causes an explosion.
Maybe in a fanfiction, Harry could be framed for something he didn't do, and Snape, believing the false accusations, smacks him in a moment of anger. This could then lead to a series of events where Harry tries to prove his innocence and Snape has to come to terms with his hasty action. Another plotline could be that Snape is under the influence of a dark spell that makes him act out of character, and Harry just happens to be in the wrong place at the wrong time and gets smacked.