Well, it could be that Hermione is under a spell when she marries Rowle. The fanfiction could then revolve around how she breaks the spell and what she does after realizing she's been forced into this marriage. She might find that Rowle has some redeeming qualities despite the situation, and they could either part ways amicably or decide to work on a real relationship.
The wizarding community would likely be in shock. Hermione is known for her association with Harry and Ron, and for her fight against the dark side. Marrying Rowle, who might be associated with dark magic, would make many question her motives. Some might think she's gone to the dark side herself, while others might suspect there's more to the story than meets the eye.
She could save him by appealing to his better nature. Hermione is known for her intelligence and ability to reason. So, she might talk to Thorfinn Rowle, convince him to turn away from whatever dark path he was on that put him in danger in the first place. By showing him an alternative way, she could in a sense'save' him from his own self - destructive actions.
Maybe he's attracted to the unique interpretations of Hermione's character in the fanfiction. Different fanfictions might show different sides of her, like her intelligence in new and exciting ways.
One common theme is the idea of redemption. Often, Rowle is depicted as a character in need of redemption, and Hermione plays a role in that process. For example, she might help him see the error of his ways.
He could be into the ones that focus on Hermione's academic achievements. Maybe stories where she discovers new spells or makes great contributions to the wizarding world's knowledge.
Obviously, Harry Potter could be the one to save her. He has a strong bond with Hermione and is very brave. He'd stop at nothing to rescue his friend. He might use some of his unique magical abilities, like his connection to Voldemort, to find her.
One great Hermione and Rowle fanfiction is 'The Unlikely Alliance'. It explores an alternative universe where Hermione and Rowle are forced to work together on a dangerous mission. Their initial distrust gradually turns into mutual respect as they face numerous challenges. Another interesting one is 'Rowle's Redemption'. In this story, Rowle has a change of heart due to Hermione's influence and they end up on a path of changing the wizarding world for the better.
There are probably some Thorfinn Rowle and Hermione romance fanfictions, but they may be hard to find. Since Thorfinn Rowle is a relatively minor character in the original series, not many authors might be inspired to write about this pairing. But if you search specifically for it on major fanfic platforms, you might get lucky.
Rowle is another main character. He's usually a dark wizard, so in this fanfiction, he could be a complex character with his own motives and backstory that Hermione has to deal with.