I haven't come across a 'snape is sirius slave' fanfiction myself. Fanfiction themes can be very diverse, but this one seems rather unorthodox and might not be as common. However, the world of fanfiction is vast, and new works are being created all the time, so it's possible that someone has written it, but it may be difficult to find.
One common theme is redemption. Many fanfics explore the idea of Snape or Sirius finding a way to redeem themselves in the eyes of the other. For example, Snape might do something heroic to earn Sirius' respect.
In some fanfictions, Sirius might be portrayed as delusional about Snape due to their long - standing enmity in the original story. Sirius has always suspected Snape of being a Death Eater, and this suspicion could be exaggerated in fanfiction, making him seem delusional.
Yes, there are some out there. You can start by looking on fanfiction websites like Archive of Our Own. Just search for the tags related to Snape being Sirius' master and you might find some interesting stories.
PotterFanatic22 is another excellent author. Their fanfics usually feature unique plotlines involving Snape and Sirius. For example, one of their works has Snape and Sirius teaming up to solve a mystery that dates back to their school days.
One great Snape Sirius fanfiction is 'The Unlikely Bond'. It explores their complex relationship in a post - war Hogwarts setting. The author does a wonderful job of delving into their shared past traumas and how they gradually come to understand each other.
One possibility could be that Snape meets James Sirius Potter in a Hogwarts flashback scenario. Maybe James Sirius Potter discovers some old memories or secrets related to Snape through a magical object. This could lead to a whole new understanding of Snape's character for him.
In fanfiction, Sirius Black and Severus Snape often have complex relationships. Sometimes they are portrayed as rivals, with their past grudges from Hogwarts days still lingering. Their different loyalties, Sirius to James and the Order, and Snape to Dumbledore (while being a former Death Eater), add layers of tension.
The power of change is also a common theme. Their relationship can bring about changes in each other. For example, Sirius might make the female Snape more open - minded, and she might make him more cautious. These themes add depth to the fanfictions and make them more than just simple love stories between two characters.
Snape might be secretly pleased at first, seeing as he has no love for Sirius. But then, being a teacher, he'd probably feel obliged to report it. He also might use this situation to his advantage against Sirius in the wizarding politics within Hogwarts.