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.
Perhaps in some 'harry marries snape fanfiction', there could be a magical accident that binds their fates together. At first, they resist the connection, but as they get to know each other better, they find common ground and fall in love, leading to marriage. This could involve a lot of emotional turmoil and self - discovery for both characters.
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.
One possible plotline could be that Harry discovers Snape's memories early on. Overwhelmed by the truth of Snape's love for his mother and his selfless sacrifices, Harry hugs Snape as a sign of immediate forgiveness and gratitude. Another could be in a post - war scenario where Snape is still alive, and Harry, now a more mature wizard, reaches out to Snape to heal old wounds.
Well, a plotline might involve a power - hungry villain who captures Tonks and makes her his slave. This could lead to a story of her resistance from within, trying to undermine the villain's plans while pretending to be compliant. And Harry could be drawn into this situation when he realizes what has happened to Tonks. Or perhaps there's a political plot where Tonks is made a slave as a form of punishment for her family's actions, and Harry gets involved in the political intrigue to free her.
One possible plotline could be Harry realizing his 'slave' status and trying to break free. He might discover some dark secrets about Dumbledore that led to this situation. Another could be Harry being forced to do Dumbledore's bidding in dangerous and unethical tasks, all while struggling with his own sense of self - worth.
One possible plotline could be that it's the result of some dark magic gone wrong. Maybe a spell was cast on Harry by an enemy of Snape's to embarrass him, and now they have to deal with the consequences together. Another could be that it's part of a prophecy that no one expected, and they must figure out how it fits into the wizarding world's future.
One possible plotline could be Harry being sold due to some dark magic plot against him. Snape then has to deal with this unexpected situation and protect Harry from further harm. Another could be that it's a part of a deal to save the wizarding world, and Snape and Harry must learn to work together in this strange new dynamic.
Another plotline could be that there is a spell or a magical occurrence that forces Snape and Harry to be close to each other. Initially resistant, Snape eventually gives in and cuddles Harry. This could then be the start of a journey where they learn to trust and rely on each other more.
The letter might be a summons to a secret wizarding council. Harry Snape's journey to deliver it could involve him facing his own inner demons as well as external obstacles. He might have to pass through dangerous terrains and encounter magical beings that are either trying to stop him or help him depending on their own agendas. As he gets closer to his destination, he discovers more about the true nature of the council and his role in the wizarding world.