A common plot is that Snape uses his position as a double agent to his advantage. He gets close to Voldemort and then at a key moment, he uses a hidden weapon or spell to injure him. For example, he might have a wand with special properties that he uses against Voldemort. This is popular because it plays on Snape's duplicitous nature.
There's a fanfic where Snape takes advantage of a power struggle within Voldemort's ranks. He poisons Voldemort's drink during a meeting, which causes Voldemort to become severely ill, an injury in a sense. Snape does this to try and disrupt Voldemort's plans and give the Order of the Phoenix more time to prepare their defenses.
Some plots might involve Snape being forced to confront his new - found relation to Voldemort in the middle of the wizarding war. For example, he could be captured by Voldemort's forces and Voldemort reveals the truth to him. Then Snape has to decide whether to embrace this new identity or fight against it. There could also be a plot where other characters like Harry and Hermione suspect Snape's relation to Voldemort and try to prove it.
One popular trope is the 'enemies - to - reluctant - allies'. Given their history of animosity, it's interesting to see them pushed into a situation where they have to cooperate. Another is the 'redemption arc' for Snape, and Sirius being a part of that in some way, either by initially opposing it and then coming around, or being a catalyst for Snape's change.
In some fanfictions, Snape might be working with Voldemort because he has been blackmailed. Voldemort could have found out some deep, dark secret about Snape and is forcing him to cooperate. This could be related to Snape's past or something he has done that he doesn't want to be made public.
Harry Potter often features prominently. His relationship with Snape is complex in the original series, and in fanfiction involving Snape and Voldemort, his role can be further explored. For example, how he reacts to Snape's interactions with Voldemort and whether he can figure out Snape's true motives sooner.
A common theme in slash fanfiction about Snape is his emotional journey. Writers often delve into his feelings of love, loss, and regret. For example, his love for Lily is a central aspect that can be explored further. Some fanfictions might show how he copes with her death over time, and how it affects his relationships with other characters. Also, the theme of his struggle between the light and the dark side of magic is often revisited, but with a new twist in the context of slash fanfiction, like how his relationships influence this struggle.
In Severus Snape family fanfiction, the theme of identity is really popular. Severus' family can play a big role in shaping his identity. For instance, if his family has a long history of being dark wizards, how does he deal with that? Another theme is family curses or blessings. Some fanfictions explore if there are any magical curses or blessings that have been passed down in the Snape family. And family loyalty is also a significant theme. Whether Severus stays true to his family values or breaks away from them is often a central plot point in these stories.
A common trope is the 'Injury Reminder'. In these fanfictions, Snape's limp serves as a constant reminder of a past battle or dark encounter. It adds a sense of mystery and danger to his character.
Power. Voldemort might see Snape as a means to gain more power. Snape is highly intelligent and skilled in the Dark Arts. By caring for him, Voldemort could be ensuring that Snape remains loyal and continues to use his skills for Voldemort's benefit.
There are also fanfictions where Snape time travels and gets involved in the founding of Hogwarts era. This is a very creative plot as it combines Snape's character with the ancient history of the wizarding world. He might use his modern - day knowledge to influence the founding principles of Hogwarts or have interactions with the founders themselves, which completely changes the course of wizarding history.