Yes, it can be a common theme. In many Harry Potter fanfictions, Harry has a strong connection to Hogwarts. Since he has so many adventures there and it becomes like a second home to him, the idea of him 'owning' it in some metaphorical sense, like having a special influence or a deep-rooted place in the school's future, is a popular concept among fanfiction writers.
I don't think it's a common theme. The Hogwarts Express is often a setting for stories about students going to Hogwarts, their first encounters, and the start of their magical adventures. While there may be some outlier fanfictions that explore other themes, the idea of 'lemon' (if we're talking about adult - like content) being dominant seems very uncharacteristic of the general body of Hogwarts Express fanfiction.
I'm not sure if it's extremely popular, but it has the potential to be. It offers a new perspective on the Harry Potter universe. Some fans might be really attracted to the idea of Harry having that kind of power and control over Hogwarts, while others may prefer more traditional storylines. However, there are likely a number of fanfiction pieces out there exploring this theme.
Maybe a jealous student from Slytherin who wants to get back at Harry Potter and his friends. This student discovers a dark and forbidden spell that creates a bondage effect. They start using it on the Hogwarts Express to cause chaos and make Harry and his friends look bad. They see themselves as an antagonist because they feel wronged by Harry and his group in some way, and this is their way of getting revenge.
Yes, it could be. Hogwarts is a very important place in the Harry Potter world, and Harry Potter is the main hero. So it makes sense that protecting Hogwarts by Harry is a popular theme.
Many fanfictions explore loss by looking at how the characters' relationships change. After Sirius's death, Harry might become more withdrawn from his friends, or Hermione and Ron might struggle to reach out to him. The way these relationships shift due to the loss is an important aspect. Also, the fanfictions might show how the memory of Sirius lingers. His presence in the characters' minds, through dreams or just random memories that pop up, shows that even though he's gone, the loss is still very much felt.
Yes, it can be. Many fanfiction writers like to explore unique perspectives, and having Hogwarts as an observer or an active part in Harry's life stories can be very appealing. It allows for a different take on the familiar storylines.
It shows how the abuse affects Harry's behavior. He might be constantly on edge, always looking over his shoulder. This is a sign of the trauma he's endured. The fanfiction may also explore how he has trouble trusting people, which is a common result of trauma.
One possible plotline could be that some dark magic has been cast on the Hogwarts Express and students start to find themselves in strange bondage - like situations, where they are restricted in their movements but have to figure out how to break free using their magic skills and knowledge. Another could be a more prank - based scenario where a group of students accidentally cast a spell that creates a bondage - like effect during their journey on the Hogwarts Express and they have to reverse it before the teachers find out.
There could be a fanfiction titled 'Draco's Hidden Shame'. In it, Draco is forced into a situation where he is made to feel ashamed about something related to his body, like a scar or a mark, and it's set in the Hogwarts environment. It might show how he overcomes this shame with the help of his friends or through his own self - discovery.