It's quite unique. In the official story, Harry and Snape are on opposite sides in many ways. Snape is often seen as a strict and sometimes unlikable figure to Harry. For a fanfiction to have them kiss in the Great Hall is very different. Fanfiction writers might be trying to explore a different side of their relationship, perhaps a side where they overcome their differences in a very unexpected way.
Well, it could be for the sake of exploring an alternate reality. The Great Hall is a place full of magic and tradition in the 'Harry Potter' world. The writer might want to create a scenario where the usual boundaries between Harry and Snape are broken. It could also be a way to show a more complex side of their personalities. Maybe Snape has a side that is attracted to Harry's courage and Harry sees a different, more vulnerable side of Snape, leading to this kiss.
Sure. There are fanfictions that revolve around the Great Hall's architecture and its historical significance in the wizarding world. Some stories might explore how students interact with the magical elements of the Great Hall, like the floating candles or the enchanted ceiling. These fanfictions add depth to the world of 'Harry Potter' without any inappropriate content.
There could be a corrupt Hogwarts teacher who has been secretly practicing dark magic. This teacher, perhaps a Potions or Defense Against the Dark Arts teacher, has been influenced by the allure of power. They see Harry as a threat to their new - found power and decide to torture him in the Great Hall to send a message to the other students that they are in charge now.
It could be due to extreme exhaustion. Harry has been through a lot in his adventures. He might have been overusing his magic, perhaps in some secret training or in trying to protect Hogwarts from an unseen threat, and his body just couldn't take it anymore, causing him to faint right there in the Great Hall.
In this fanfiction, Snape might finally expose the Dursleys' ill - treatment of Harry. He could use his magic to intimidate them into treating Harry better. Maybe he'd discover some dark secrets about the Dursleys that they've been hiding all along.
There are versions where Snape reacts defensively. Let's say Lily confronts him about his actions that seem to be harming her or her friends. He would immediately start justifying himself, listing all the reasons why he did what he did, trying to make Lily see his point of view, even if deep down he knows he might be in the wrong.
The Dursleys would probably be more frightened. They are muggles and Snape is a powerful wizard. The idea of a strange man in black robes suddenly appearing and using magic would be terrifying for them.
One popular theme is the Sorting Ceremony. Fanfictions often explore the emotions and experiences of new students during the Sorting in the Great Hall. Another theme is the feasts. Writers might describe in detail the delicious magical food served in the Great Hall during these feasts and the interactions between characters.
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.