Obviously, Harry Potter and Snape are the main characters. Harry, with his courage and leadership, and Snape, with his complex past and skills. But other characters like Hermione and Ron could also play important roles. They might be the ones who first notice the change in Snape's behavior towards Harry and help Harry deal with this new situation.
One possible interesting plot could be Snape having to follow Harry Potter's lead in a dangerous mission. Maybe they are in a situation where Snape has no choice but to trust Harry completely, which is a big change from their usual relationship in the original books.
At first, it could be very tense. Harry might be reluctant to submit, but as time goes on, he starts to see Snape in a different light. This could lead to a grudging respect. For example, Snape might show Harry some advanced potions techniques that no one else has taught him, which makes Harry realize Snape's value.
Perhaps in this fanfiction, Harry submits to Snape in order to gain access to some important information. Snape might be in possession of secrets about Harry's family or his own powers that he refuses to share unless Harry shows complete obedience. So, Harry decides to submit to Snape's demands in order to uncover these mysteries that are crucial for his journey as a wizard.
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.
In some fanfictions, Snape might use his advanced knowledge of potions to create a special concoction to heal Harry. He could draw on his skills in Herbology as well to find rare ingredients for the potion.
One possibility could be that Harry, out of a sense of duty or some form of magical coercion, decides to follow his uncle's every order in order to protect his aunt and Dudley from a new magical threat. He might suppress his own desires and instincts to keep them safe.
It could be that after a series of events that make Harry question his own judgment, he submits to Dumbledore's authority. For instance, if there are false prophecies or misinformation floating around that lead Harry astray. Dumbledore, with his greater experience and knowledge, helps Harry see the truth. So Harry decides to submit to Dumbledore in order to get back on the right track and protect Hogwarts and the wizarding world.
The main characters would be Harry and Dumbledore. However, the antagonists could also be important. If there is a new dark force in the fanfiction, they would be the ones Harry and Dumbledore are up against. Voldemort's remaining followers could also be part of the equation. Their actions could be what drives Harry to submit to Dumbledore's leadership, as they pose a significant threat to the wizarding world and Harry realizes he needs Dumbledore's guidance to face them.
One more to consider is 'Snape: Beyond the Surface'. In this story, the author explores Snape's relationships with other characters like Dumbledore and McGonagall. It shows how his interactions with them were not always as straightforward as they seemed in the original books and adds more depth to his character.