One popular plotline could be Sasha being a new student at Hogwarts who catches Snape's eye in a Potions class. Maybe she has a natural talent for Potions that he can't help but notice. Another could be that Sasha is involved in some sort of mystery at the school, and Snape has to reluctantly work with her to solve it, all while their relationship develops in unexpected ways. A third plotline might be that Sasha is an outsider to the wizarding world but has a unique power, and Snape is tasked with training her, leading to a slow - burning romantic connection.
Well, often in these fanfictions, a common plotline is that Sasha is a healer in training. Snape gets injured during some sort of skirmish or accident at Hogwarts. Sasha is the one assigned to take care of him. As she nurses him back to health, they start to develop feelings for each other. Another plot is that Sasha is a Muggle - born witch who Snape initially despises, but as he gets to know her better, he realizes she's different. And then there's the plot where Sasha is a time - traveler who comes to Hogwarts in the past, and she meets Snape. Their relationship then unfolds in a very different time period.
Some plotlines include Sasha being a part of an underground resistance group against the Dark Lord, and Snape has to keep her identity a secret while also falling for her. There could also be a plot where Sasha is a magical creature expert, and she and Snape bond over their shared love for the more esoteric aspects of the wizarding world. Another interesting one is when Sasha is a descendant of an old wizarding family, and she comes to Hogwarts with a lot of secrets. Snape, being the nosy and observant man he is, gets drawn into her world and their relationship blossoms from there.