One possibility could be that the snakes are magically bound to protect Harry. Maybe they are under an ancient spell that makes them his guardians. For example, they could form a protective barrier around him whenever he is in danger, like when Voldemort or his Death Eaters are near.
It could be a dark wizard who wants to break the bond between Harry and the snakes. This wizard might see Harry's protection by the snakes as a threat to his own power or plans.
Well, perhaps in this fanfiction, Harry discovers he has an affinity for snakes early on. Maybe he's able to communicate with them in a deeper way than Parseltongue allows. The snakes could train him in stealth and survival skills, like how to move quietly in the Forbidden Forest or find hidden passages. They could also teach him some unique magic that is related to the ancient magic of serpents.
Maybe Harry discovers a baby snake in the Forbidden Forest and decides to take it under his wing. He could use his Parseltongue ability to communicate with it and keep it hidden from the other students and teachers.
Maybe Harry discovers a new type of snake with magical abilities. For example, it could be a snake that can turn invisible at will. This would be really cool as it could add a new dimension to the wizarding world's magical creatures.
One interesting plot could be the protagonist, as the King of Snakes, using snake abilities to solve mysteries in Hogwarts. For example, being able to communicate with the snakes in the Forbidden Forest to find out about hidden secrets or dark magic.
Perhaps some traditionalist wizards in the Ministry of Magic. They might view Harry's training by snakes as a form of dark magic practice, even if it's not. So they become the antagonists, trying to arrest Harry or stop his training in some way.
I'm not entirely sure as there could be many different interpretations. It might be about how Harry Potter is safeguarded by the champions in some sort of alternate scenario within the fan - created story.
In this fanfiction, perhaps it's not just physical protection. Gringotts could be protecting Harry Potter in a more symbolic or magical sense as well. They might be guarding his family's legacy, which is tied to certain items or secrets in the vaults. This could lead to a whole series of adventures where Harry has to work with the goblins to uncover his true heritage and at the same time, stay alive while being protected by the might of Gringotts.
Some fanfictions might introduce new and original snakes. For example, a sentient snake that can speak multiple languages and acts as a guide for lost wizards in the Forbidden Forest. This kind of unique snake can become popular as it adds new elements to the familiar Harry Potter universe.
In Harry Potter fanfiction, children can be protected in various ways. For example, the magical world often has protective spells and charms like the Fidelius Charm which can hide a person or place. Fanfiction authors might use these concepts to safeguard the child characters. Also, the friendship and loyalty among the characters act as a form of protection. When Harry, Ron, and Hermione are together, they look out for one another.