One possible plot could be that Hermione, in her quest for knowledge about the dark arts practitioners, comes across Thorfinn Rowle. She might initially be studying him as part of her research on Death Eaters. And as she tries to understand his motives and past, she gradually gets to know him on a more personal level.
You can start by looking on major fanfiction platforms like Fanfiction.net or Archive of Our Own (AO3). On Fanfiction.net, use the search bar and type in 'Hermione gets to know Thorfinn Rowle'. AO3 also has a great search function. Just be sure to check the tags and summaries to find the ones that match what you're interested in.
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.
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.
Yes, there is. In some fanfictions, they might be put in a situation where they have to work together. For example, they could be forced to collaborate to solve a mystery within the wizarding world that affects both of their interests. This can lead to some interesting character development and relationship exploration between Hermione and Thorfinn.
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.
It's hard to say which ones are 'good' without actually reading a bunch. However, on AO3, stories with a high number of kudos and positive comments are likely to be well - received. You could start with those and see if they match your idea of a good story. Also, some stories might be part of a series, so exploring related works by the same author could lead you to more engaging content.
Redemption is a big theme. Since Thorfinn has ties to the dark side in the original story, many fanfictions explore the idea of him being redeemed through his interactions with Hermione. Also, the theme of unexpected alliances can be found, as they come from different 'worlds' within the wizarding universe.
Perhaps the fanfiction focuses on Hermione's research into some dark magic spells and she finds out that Rowle has some knowledge about them. So, she has to get close to him to learn more. In the process, they might develop an unlikely friendship. Rowle could start to see the value in the light side of magic through Hermione's influence.
One possible plot could be that Hermione's friends, like Harry and Ron, immediately start a rescue mission. They use their knowledge of magic and Hogwarts' secrets to track down Thorfinn Rowle. Another plot could involve Hermione using her own intelligence and magic skills while being held captive to outwit Rowle and escape on her own. Maybe she discovers some dark secrets about Rowle during her captivity that she can later use against him.