Severus might be developed as a more complex and empathetic character. By saving Hermione from Ron, he shows a side of himself that is protective and caring, which is different from his usual cold exterior. This could be a turning point in how the other characters view him.
Another way Severus could save Hermione is by using his knowledge of the Dark Arts in a positive way. If she was hurt by a Dark spell from Ron (in the context of the fanfic), Severus might be able to counteract the spell's effects. He has extensive experience with Dark Arts spells, so he could figure out what spell Ron used and then use a counter - spell to reverse the damage.
There might be some kind of misunderstanding. For example, Ron could have been misinformed about Hermione's actions. He thought she betrayed him in some way, like sharing Gryffindor's secrets with Slytherins, so he lashes out at her.
One possible plot could be that Ron is under the influence of a dark spell that makes him act aggressively towards Hermione. Severus, being observant, notices this and steps in to protect Hermione. He uses his potions knowledge to create an antidote for the spell on Ron.
Well, in fanfiction, anything can happen. Hermione leaving Ron for Severus might be a way for writers to show Severus in a different light. Perhaps they see Severus as someone who has been misunderstood and Hermione, being the intelligent and perceptive character she is, could be the one to recognize his true worth. But it also shakes up the core relationships we knew from the original series, which can make some fans uncomfortable.
One popular trope is the 'enemies to lovers' where Severus initially dislikes Hermione but gradually falls for her. Another is the 'teacher - student relationship' dynamic which adds an element of forbidden love. And the 'redemption arc' for Severus through his relationship with Hermione is also common.
Severus might start off as just fulfilling a sense of duty. He has always been a complex character. But as he saves Harry from the Dursleys, he begins to see Harry in a new light. He realizes that Harry is not just James Potter's son but an individual with his own struggles. This could lead to him becoming more empathetic towards Harry, gradually shedding some of his cold exterior.
It should start with mutual respect for each other's intelligence. Snape respects Hermione's academic achievements and Hermione respects Snape's vast knowledge of magic. Then, they could be thrown into a situation where they have to rely on each other, like a dangerous magical experiment gone wrong.
For Hermione, in fanfic she might be more daring than in canon. She could be more willing to take risks without thinking too much about the rules, especially when it comes to her relationship with Scorpius and Severus. Scorpius, who is often seen as a bit of an outsider in the official stories, could be developed into a more confident character, perhaps even leading the group at times. Severus, instead of his usual brooding self, could be more open and less bitter, sharing his knowledge more freely.
I'm not sure if there is such a specific fanfiction. However, in the general context of Hermione and Severus fanfiction, their relationship is often complex. In some fanfictions, they might develop an unexpected bond through shared intellectual pursuits or mutual understanding of the wizarding world's deeper secrets.
One reason it could be popular is the allure of the forbidden or unexpected. Ron and Hermione were a well - known couple in the original series, so the idea of Hermione leaving him for Severus is a huge deviation. It also gives a chance to delve deeper into Severus' character. Fans might be interested in seeing how he would react to such a relationship.