Perhaps a plotline where Sirius Black discovers a hidden prophecy related to Harry Potter that requires Harry to submit to Sirius in a series of magical trials. These trials could be designed to unlock Harry's true potential. Or it could be a plotline where the wizarding world is in chaos, and Sirius, with his knowledge and experience, convinces Harry that submitting to his leadership is the only way to restore order. This could involve Harry following Sirius' orders in battles, in diplomatic relations with other magical creatures, and in reforming the Ministry of Magic.
One possible plotline could be that Harry has done something dangerous in the wizarding world, like sneaking into the Forbidden Forest without permission. Sirius, as his godfather, decides to give him a mild spanking as a form of discipline while also explaining why his actions were wrong and how they could have put him in great danger.
Yes, it could potentially be a popular genre within the fanfiction community. Fanfiction allows for a great deal of creative license, and fans often like to explore different aspects of character relationships. The concept of Harry Potter submitting to Sirius Black might attract those who are intrigued by the mentor - mentee relationship between them taken to an extreme level. It could also be popular among fans who enjoy stories with elements of loyalty, trust, and power play. However, it's also likely to be controversial as it can be seen as changing the fundamental nature of the characters too much for some readers.
One common plotline is Sirius Black escaping from Azkaban earlier and secretly training Harry in advanced magic. Another could be Sirius and Harry going on a hidden adventure to uncover more about Harry's parents. There are also stories where Sirius plays a more active role in the fight against Voldemort at Hogwarts, using his knowledge of the Dark Arts and his Animagus form to their advantage.
One possible plotline could be that Sirius Black has been under some dark magic influence. Maybe a powerful dark wizard has managed to manipulate his mind, making him believe Harry Potter is a threat to him or the wizarding world in some false way. This could lead to a series of events where Harry tries to prove his innocence and figure out what's wrong with Sirius. Another could be that there's a misunderstanding based on some prophecy or false information spread within the wizarding community. Sirius, being impulsive, jumps to conclusions and decides to disown Harry without fully understanding the situation.
One interesting plotline could be how Sirius helps Harry discover more about his parents in a more personal way. Since Sirius was James Potter's best friend, he could share countless stories and memories that Harry never knew. Another plotline might involve Sirius teaching Harry some advanced magic that he wouldn't learn at Hogwarts. This could give Harry an edge in dealing with the various threats he faces. Also, their relationship could be explored in depth, showing how Sirius becomes not just a guardian but also a true father figure to Harry.
One possible reason could be a misunderstanding. Maybe some false information was spread about Harry that made Sirius believe he had done something unforgivable. For example, if there were rumors that Harry was involved with dark magic in some way, Sirius, who has a strong aversion to the dark side, might be quick to disown him.
Well, perhaps the fanfiction could start with Harry finding an old family heirloom or a mysterious spell that transforms him into Warlock Black. Then, he might have to keep this a secret from his friends at first. As the story progresses, he could find that being Warlock Black gives him access to ancient magic that the Dark Lord is also after. So he has to race against Voldemort to master this magic and use it for good. Also, his relationships with the members of the Black family, both the good and the bad ones, could be a major part of the plot.
Maybe Sirius wants to train Harry himself. In fanfiction, he could believe that he has a better understanding of the Dark Arts and how to fight Voldemort compared to the professors at Hogwarts. So, he kidnaps Harry to start his own special training regime away from the school.
In some fanfictions, it could be due to dark magic influence. A spell or a dark force might have corrupted Sirius's mind, making him turn against Harry.
A plotline might involve Sirius and Harry living together and Sirius trying to give Harry a normal family life. He could cook for Harry, help him with his homework, and they could have a pet together. Maybe they get a Kneazle, and Harry learns to take care of it with Sirius' guidance. This would be a contrast to the cold and often neglectful treatment he received from the Dursleys. And as Harry grows, Sirius could introduce him to his old friends like Remus Lupin in a more family - like setting, strengthening Harry's support system.