They might start off as enemies. Sirius doesn't get along with his family, so he may not want the Marauders to have anything to do with Regulus. But sometimes, they could end up working together for a common good.
Sirius would probably be angry at first in the fanfiction. He left his family and Regulus was still part of it. But as the story progresses, he might start to feel some regret or longing for his brother. James would be supportive of Sirius' emotions, and Remus would be the voice of reason, analyzing Regulus' actions and motives.
One common theme is loyalty. Sirius is a very loyal character, and this often plays a big part in his love stories. His loyalty to his friends and his love interest is a key aspect. Another theme is the struggle against the dark times. The Marauders era was filled with danger, and his love story often has to deal with that.
There are fanfictions where they meet in a dangerous situation. For example, in a Forbidden Forest encounter. The Marauders are exploring, and Regulus has gotten lost. They initially suspect each other of ill intentions but then realize they need to work together to get out. And there's also the type where they meet at a pure - blood event, which challenges their different beliefs about blood purity and friendship.
Regulus Black was a member of the Black family. He was initially a Death Eater, following in the footsteps of Voldemort. However, he later had a change of heart. He discovered Voldemort's Horcruxes and tried to destroy one. He left a note for Kreacher, his house - elf, asking Kreacher to lead Harry Potter to the locket Horcrux. His actions showed that he was brave in the end, trying to make up for his past mistakes in following the Dark Lord.
Another good one is 'The Marauders' by an anonymous author. It focuses a lot on their pranks and adventures. It has a great sense of humor that really captures the spirit of the Marauders. You'll find yourself laughing out loud at their antics and also feeling the bond between the friends.
One great 'oc marauders era fanfiction' is 'The Marauders' Daughter'. It has a really engaging plot where the main OC is the daughter of one of the Marauders. The story does a wonderful job of weaving in the original Marauders' story while also giving the OC a unique and interesting backstory.
They are likely to be horrified. James, for example, would probably be extremely angry that his friend had to endure such a thing. He might immediately vow to protect Sirius from any further harm. Remus, being more empathetic, would be there to comfort Sirius and offer his support in a more quiet and understanding way. Peter, though not as strong - willed as the others, would still be shocked and follow the lead of James and Remus in trying to make Sirius feel better.
Archive of Our Own (AO3) is a great platform for 'Marauders meet Regulus' fanfiction. It has a large collection with various interpretations of their meetings. Fanfiction.net is also a well - known platform where you can find such stories. There are different tags and filters to help you find exactly what you're looking for.