One key element is Hermione's motivation. Usually, it's her sense of justice and her knowledge of Regulus' story that drive her to save him. For example, in many fics, she realizes that Regulus was trying to do the right thing in the end and she can't let his sacrifice be in vain.
One common theme is family loyalty. Sirius is driven by his love for his brother and his sense of family duty to save Regulus. It shows that no matter how estranged they may seem, family bonds are strong.
Yes, there are likely fanfictions out there with such a plot. Fanfiction writers often come up with creative and alternative family relationships for characters. They might explore how Hermione's story could be different if Regulus were her father, perhaps delving into how her magical abilities or her place in the wizarding world might be affected by this new family connection.
Hermione in these fanfictions can be a bit of a maverick. She might break some rules to get to the truth about Regulus. Her loyalty to her friends is still there, but she's also driven by a need to uncover the mystery around him. She's not afraid to question the status quo of the wizarding world's view of Regulus. And she's always learning, adapting to new situations that arise from her interactions with him, whether it's about ancient magic or the politics of the wizarding families.
Fanfiction.net is also a good place. There are many different fanfics on there. However, the tagging system might not be as refined as AO3, so you may need to do a bit more sifting through the results to find the good Hermione and Regulus ones.
There's also 'A Different Kind of Magic'. In this fanfic, Hermione discovers a hidden diary of Regulus which leads to an adventure where they form an unexpected alliance. It's full of mystery and character exploration.
One great Hermione and Regulus time travel fanfiction is 'A Time to Heal'. It beautifully weaves their stories together as they navigate through different time periods. The author does an amazing job of character development, making their relationship seem very natural.
In this fanfiction concept, it could lead to a whole new set of character dynamics. Hermione's intelligence could be seen as an inheritance from the Black family's cunning. For example, she might discover some long - lost Black family magic that helps her in her adventures at Hogwarts. It could also create interesting interactions with Sirius Black, who was Regulus' brother. Maybe Sirius would be initially shocked but then grow to be very protective of Hermione.
Well, it's because they are two very interesting characters from different generations. Regulus is this mysterious figure from the past with a complex backstory, and Hermione is a brilliant, modern - day witch. Combining them creates a unique dynamic that fans love to explore.
Another one to check out is 'Saving Regulus: Sirius's Mission'. This fanfiction delves deep into the wizarding world's politics and how Sirius has to navigate through it all to reach and save his brother. It has some really interesting characterizations and a great plot line.