Obviously, Harry Potter and Sirius Black are the main characters. Harry is the time - traveler, and Sirius becomes his mate in this new timeline. Other characters like Dumbledore are likely to also play important roles, as he is always involved in matters related to Harry and the greater good of the wizarding world.
One possible plot point could be Harry using his knowledge of the future to save Sirius from his untimely death. For example, he might prevent Sirius from going to the Ministry of Magic that fateful night. Another could be Harry befriending Sirius even earlier in time and changing the course of their relationship. Maybe they end up working together on some secret mission against Voldemort much sooner than in the original timeline.
One great fanfiction with this plot is 'Time's Redemption'. In it, Harry uses a powerful time - turning spell he discovers in an ancient book. He has to face many challenges in the past, like avoiding detection from the Ministry's time - turner regulations. Another good one is 'Back to Save Sirius'. Here, Harry teams up with Hermione and they use a secret time - travel device left by Dumbledore.
There could be unforeseen magical consequences. Time - traveling is a tricky business in the wizarding world. By changing Sirius' fate, Harry might accidentally create new problems. For example, the balance of magic in certain areas could be disrupted. There might also be changes to the Hogwarts' community. Students and teachers who were affected by Sirius' situation in the original timeline might act differently.
In such a fanfiction scenario, Harry could use a Time - Turner. He'd have to be extremely careful not to disrupt the timeline too much. First, he'd need to figure out the exact moment in time when he could prevent Sirius from getting into the dangerous situation. Maybe he'd have to go back to before Sirius was framed for the murders. Then he could expose the real culprits and keep Sirius out of Azkaban.
Definitely Hagrid. Hagrid has always been around Hogwarts and has a connection to the students. Sirius could interact with Hagrid if he goes back in time. Maybe Hagrid could help Sirius in some way, like providing shelter or information. And Bellatrix Lestrange could also be in the picture. Since she is part of the Black family, Sirius might have to deal with her in the past, either to try to change her or to stop her from doing something evil.
Typically, these fanfictions focus on a strong bond between Harry and Sirius. It often shows a deep emotional connection, like in 'A Bond of Trust' where Harry confides in Sirius about his fears regarding Voldemort.
One common plot point is the discovery of a way to time - travel. It could be a new spell, a hidden device, or something left by a powerful wizard. Another is the struggle to avoid changing too much of the past. Harry usually has to be very careful not to disrupt the timeline while trying to save Sirius. For example, he can't accidentally stop something important from happening that is related to the future.
Well, sometimes in these fanfictions, a plot twist might be that Sirius goes back in time and discovers a hidden truth about his family. Maybe he finds out that there was a secret alliance or a hidden enemy within the Black family that was influencing events all along. This could completely change his perception of his family's past and his place in it. Also, he could go back in time and end up being involved in a different kind of magic that was lost in his original time, like an ancient form of magic that has the power to either save or destroy the wizarding world.
Yes, there are quite a few out there. You can start by looking on popular fanfiction websites like Archive of Our Own. Just search for the key phrase 'Harry is Sirius submissive mate' and you'll likely find some interesting stories. Some may focus on their unique bond in the context of the wizarding world, exploring how their relationship could develop in different scenarios.