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.
In this fanfiction, under Snape's desk might be a collection of old parchments that hold the secrets of Snape's past. Harry, while snooping around, discovers these parchments. They reveal that Snape had a connection to Harry's family long before he became a teacher at Hogwarts. This discovery changes Harry's perception of Snape completely.
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.
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.
One possible plot could be that Harry and Snape are magically forced to hide under a table during a chaotic event in Hogwarts. Maybe there's a rampaging magical creature loose in the classroom and they have to stay hidden. Harry is initially distrustful of Snape as always, but Snape ends up using his skills to protect Harry from the creature's detection, leading to a begrudging respect between them.
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.
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.
One possible plot could be that Harry accidentally breaks the desk during a spell - casting practice in Snape's class. Snape then uses this as an opportunity to give Harry extra detention and during that time, they start to have a more in - depth conversation about Harry's future at Hogwarts and Snape's past with Harry's father. This could lead to a new understanding between them.
In this fanfiction, it could also be that Hermione is forced under the desk as part of a mystery. There could be some hidden secrets or clues under the desk that she needs to discover, but she's being forced to stay there against her will at first. As she investigates, she uncovers something that has far - reaching implications for the wizarding world.
One possible plot could be that Harry discovers he has a hidden connection to Sebastian Snape, perhaps a long - lost family relation. This revelation could lead him to question his identity and his place in the wizarding world.
In a 'Harry Potter confides in Snape' fanfiction, the plot might involve Harry being tormented by visions of the future. He can't share these with Hermione and Ron as they are too worried about him already. So, he turns to Snape. Snape, with his knowledge of the Dark Arts and Occlumency, helps Harry understand and control these visions. This could lead to a new bond between them, different from their usual tense relationship.
The plot could involve Snape trying to protect his daughter from the dark forces in the wizarding world without revealing his true relationship to her. He might give her special potions or spells to help her in her adventures at Hogwarts. And as Harry grows, she might start to look more like Snape, which causes some confusion and speculation among the other characters. This could lead to a series of events where Harry slowly uncovers the truth about her father.