Perhaps there could be a stack of old, hand - written notes. These notes might contain his personal thoughts on magic, or his memories related to certain spells and potions. Snape is a very private person in the original stories, so these notes could be full of secrets. For example, they could be about his past relationships or his early experiments with magic that he doesn't want others to know about.
Obviously, Harry Potter would be one of the main characters. He's the one who is likely to be under Snape's desk for some reason. And of course, Snape himself. His desk is the central location, so he has to be involved. Also, Hermione might be there. She often helps Harry out, and if Harry gets into trouble under Snape's desk, she could be the one to figure out a solution.
There could be a lot of things like old papers, a lost pen, or some forgotten books.
It could be the Slytherins. They are often portrayed as the antagonists in the Harry Potter series. Maybe they're jealous of Hermione's intelligence and want to embarrass her by forcing her under the desk.
Well, it could be about Harry finding a hidden magical object under Snape's desk. Maybe it's a potion ingredient that Snape has been secretly guarding. Harry, being curious, decides to investigate further and gets into all sorts of trouble as a result.
Some fanfictions might suggest that there are hidden portraits under his desk. These portraits could be of his family members or people he once knew and cared about. They could be whispering advice or secrets to him when no one else is around.
One possibility is that this is a misnomer or a very tongue - in - cheek title. In the fanfiction, it could be that Harry is using some new form of magic under Snape's desk that has an effect that can be humorously described as'sucking'. For instance, he could be using a spell to draw out negative magical energy that has accumulated under Snape's desk, which is causing problems in the classroom, and the spell's action makes it seem like he's'sucking' something out.
It's possible to find a stack of papers. These could be ungraded assignments, old lesson plans, or notes from previous classes. There might also be a small wastebasket under the desk for the teacher to throw away any scraps of paper or used tissues during the day.
This description involves inappropriate content, so I can't provide relevant help.
I'm sorry, but this topic involves inappropriate and unethical content, so I can't provide relevant fanfictions.
I'd say it's quite creative in a rather odd way. The combination of characters and the strange situation described makes it stand out. It immediately grabs your attention, even if it might be a bit difficult to understand exactly what it means at first.