Disclaimer: I don't own the Harry Potter franchise. If I did, I'd be making some big bucks by selling this story somewhere instead of posting it on FFN for free.
Disclaimer 2: The idea of Snape's life debt to the now late James Potter making him a Godfather to Harry is something I found in a fanfic written by Nebresh, who gave me permission.
Author's Note: This is an AU fanfic where Severus Snape didn't become a Death Eater and someone else overheard a bit of the prophecy and relayed it to Voldemort.
Chapter 1: The Secondary Godfather
Like every other potion maker in England, Severus Tobias Snape or Snivellus as some of the Marauders liked to call him was requested by St. Mungo's Hospital for Magical Maladies and Injuries for hangover potions because of the expected increase on the demand as a result of several wizards and witches drinking more than usual to celebrate Voldemort's downfall. Not that he could blame them for celebrating after all the time he's been afraid of being killed for refusing to become a Death Eater, which probably would have already happened if he didn't properly set protective wards around his shop. Upon reading the Daily Prophet, he found something that really disturbed him: his old crush was killed by Voldemort. Poor Lily. What really made Snape mad, however, was the fact she and her despicable (in Snape's opinion) husband James Potter had their whereabouts revealed to their killer by a traitor. 'I knew Black couldn't be trusted but even I didn't expect that.' Snape thought. As he kept reading the story, he learned that Potter's other friend Peter Pettigrew was killed along with some muggle bystanders by an explosion caused by Black. Now there's something even harder to believe for Snape unless it turned out Pettigrew was the Potters' real betrayer and Black was too furious care about who else got killed. Snape's main concern was Lily's son Harry Potter. With Black in prison, Pettigrew dead and Remus Lupin being a werewolf, the boy probably wouldn't have anyone to raise him except for his muggle Aunt Petunia and her husband Vernon Dursley and Severus Snape didn't expect them to be better parents to wizarding children than his own muggle father was. Snape did want to raise the son of his beloved Lily but how to take the boy in without revealing his admittedly unhealthy obsession? He soon came up with a way and wanted to hex himself for considering it.
With Voldemort gone, the Ministry of Magic was in uproar as several Aurors were bringing in every suspected and/or confirmed Death Eater. Indifferent to all of that, Severus Snape went through the Ministry entrance and stated his intention to speak to Bartemius Crouch about a long time debt he owed to the Boy-Who-Lived's family. That piqued the receptionist's curiosity and, combined with the fear she felt of Snape's glare, made her let Snape in.
"How can I help you, potioneer Snape?" The Head of the DMLE asked.
"Back when James Potter and I were Hogwarts students, he saved my life and I never had a chance to repay the resulting life debt." Snape explained. "I want to repay the debt by becoming his son's Godfather."
Crouch hated serving as a pawn for people wanting to bask on the glory of raising the Boy-Who-Lived but knew he couldn't get in the way of someone repaying a life debt. He also knew Severus Snape wouldn't claim to owe anything to James Potter unless that was the absolute truth. "Be as it is, Mr. Snape, Chief Warlock Dumbledore already found a family for the Boy-Who-Lived but I'll get in contact with them to let them know how they feel about your offer."
"Thank you, Director Crouch." Snape politely said. "By the way, when will Sirius Black be brought to the Wizengamot to answer for his crimes? I'd love to see his face one he's pronounced guilty."
"There's no need to waste time with a trial for him, Mr. Snape." Crouch answered with a "that's-final" tone. "He's obviously guilty."
"Aren't you afraid his family will use the lack of a trial to get him off on some technicality?" Snape asked out of worry. "They're quite rich and powerful." He added, understating the Noble and Most Ancient House of Black's power.
"Wasn't he disowned?"
"Walburga Black might have drafted a will bequeathing her whole estate to her other son Regulus but I am not aware of any plans the other Blacks have for their assets." Snape stated, worrying Crouch. "For all I know, Sirius Black became his mother's heir by default upon his brother's death."
"I'll talk to Dumbledore about all possibilities so Black won't be able to invoke any technicalities." Crouch declared. "Is there anything else I can do for you, Mr. Snape?"
"This is all. Thank you." Snape said.
"In this case, please show up tomorrow for a reply regarding your request." Crouch asked.
The next morning, Severus Snape showed up and, since he had an appointment this time, didn't have to wait. "So, Director Crouch, what did Harry Potter's family have to say about my offer?"
"Mr. and Mrs. Dursley were too eager to accept your offer and practically demanded you took the boy away from them as soon as possible." Crouch answered with a frown while Snape seemed unsurprised. "If not for our insistence you should pick him up personally, I'm certain they'd force us to take the boy with us right after we informed them about you. I wonder what Dumbledore was thinking when he placed our savior with them."
"It's hard to tell what he thinks, Director Crouch." Snape commented. "Perhaps it'll be better to keep the recent development away from his knowledge for as long as possible just in case he's stubborn enough to force Harry into them again."
"I'm afraid I must agree with you, Mr. Snape." Crouch sadly replied. "Hopefully, the desire to keep Black in Azkaban will make him too worried to notice.