In certain fanfictions, there's a time - travel plot. Harry goes back in time and has to work with a younger Snape and Draco to change the future. This allows for a whole new exploration of their relationships and how they interact with each other in different time periods. There are also often revelations about the past that change the way the characters view each other.
Some fanfics have a plotline where Draco becomes a double agent, and Snape has to protect him while also dealing with Harry's suspicions. It creates a really tense and interesting dynamic between the three.
One unique plotline is when Snape has to train both Draco and Harry in a special form of magic that was lost for centuries. This brings out a whole new side of their relationships as they have to rely on each other to master the magic. It's different from the usual Hogwarts drama we see in the books.
Another plotline could be that they are all given a mysterious task by an ancient magic entity. To complete this task, they must put aside their differences. Snape and Lucius, who have a history of being on the same side but also having their own rivalries, have to work with Harry, whom they have long seen as an adversary. Draco, caught in the middle, has to choose between his family's old ways and this new alliance, which creates a lot of internal conflict and an engaging story.
Well, one unique plotline is when Draco and Harry are forced to work together on a secret mission for the Order of Phoenix. They have to put aside their differences and learn to trust each other, which is a complete turnaround from their usual rivalry in the original books.
One unique aspect is the exploration of their mentor - mentee relationship. Snape, being a complex character himself, can offer Draco different perspectives that we don't see in the original books. For example, in some fanfics, Snape shares his own experiences with the Dark Arts to warn Draco about its dangers.
One unique plotline is when Harry and Draco are magically bound to protect each other's families. It forces them to work together and Hermione is the mediator. They have to overcome their past enmity for the sake of their loved ones.
A great plotline could be Snape training Draco in advanced potions and magic that are not part of the regular Hogwarts curriculum. This could lead to Draco having unique skills that help him in the wizarding world. Also, Snape might take Draco on secret excursions to learn about the history of the Dark Arts in a way that doesn't lead him down a dark path but rather makes him understand the dangers better.
A really cool fanfiction could be set in an alternate universe where Harry, Snape, and Draco are all part of a secret society in Hogwarts. They have to keep the society's existence and purpose a secret while dealing with their own personal issues and rivalries within the group.
One great fanfiction could be 'The Unlikely Alliance'. In this story, Harry, Snape, and Draco are forced to work together during a time when Hogwarts is under a new and mysterious threat. It shows a different side of their relationships as they learn to rely on each other's strengths.
In some 'Snape Adopts Harry' fanfictions, there is a plotline where Snape and Harry have to work together to uncover a mystery at Hogwarts. This mystery could be related to the Dark Arts or some hidden secrets of the school. Along the way, their relationship evolves from a reluctant guardian - ward relationship to a more loving father - son bond. Harry might start to understand Snape's past actions and Snape, in turn, learns to open up and be more affectionate towards Harry.
One interesting plotline could be Snape realizing that Harry is not like the 'Boy Who Lived' (WBWL) as everyone expected. Maybe Harry has a different approach to magic that Snape initially scoffs at but later comes to respect. For example, Harry might be more interested in the darker arts in a way that challenges Snape's views.