Regulus lived and is a spy and teacher at Hogwarts: black brothers shenanigans. Regulus " my older brother is a moron!" Sirius, "and my little brother is a bitch." Trauma bonding. Mommy issues. Regulus being petty. Cleaning their childhood home together. Kreacher! Sirius and Severus arguing in the kitchen scene and Regulus not stopping it but treating it as entertainment. Bonding over hate of snape and love and protectiveness of Harry. Regulus teasing Sirius about Remus. Avoiding the James size elephant in the room.
yes actually i really love this but how would they interact when sirius escaped from azkaban and regulus was at hogwarts and sirius saw him a few time and didn't know if he would approach him or not, thinking regulus is a spy for voldemort when he's a double agent for dumbledore regulus not being able to believe his brother could ever be responsible for james and lily's deaths and knowing he's innocent but could never prove it or save him regulus never knowing who the black dog he fed once in hogsmeade was