Albus Dumbledore once said that 'the consequences of our actions are always so complicated, so diverse, that predicting the future is a very difficult business indeed', and he was right. The fickle nature of fate and prophecies comes up time and time again throughout the Harry Potter series. While Harry Potter became the Chosen One, there were so many different roads that the story could have taken; if Voldemort had gone after Neville instead of Harry, for instance, or if Albus hadn't invited Tom Riddle to Hogwarts.
With all of this in mind, I still reckon that the wizarding world's fate was sealed the moment the Sorting Hat yelled 'Slytherin!' after being placed on an 11-year-old Severus' head. Here's how things might have been different if Snape was never sorted into Slytherin.
Firstly, a quick disclaimer for the proud snakes out there. We know that Slytherin is a great house. It is for the smart, the ambitious and the quick-thinking, and it deserves to be respected. However, back in the day when Voldemort was growing in power and gaining followers, Slytherin's reputation became murky. Salazar Slytherin's influence on the house also encouraged certain students to carry the thoughts that 'pure bloods' were somehow better than others, from Tom Riddle - all the way up to Malfoy 50 years later.
When Snape first met Lily, the pair instantly hit it off and became very close, with Severus sharing everything with her: from what she needed to know about magic, to his own unhappy home life. But when they went to Hogwarts, everything changed. The pair were parted when the Sorting Hat placed Severus in Slytherin and Lily in Gryffindor, and while Lily thrived in her new, magical life, Snape was essentially placed in close quarters with the likes of Lucius Malfoy, Avery and Mulciber. Mulciber!
If he had been sorted into Hufflepuff, Ravenclaw or Gryffindor, perhaps Snape stood a chance at making a much safer choice of pals. And it has to be said, he was relentlessly bullied by James and Sirius, since the pair had an innate dislike of Slytherin before they even got to know Snape at all, causing him to be angry, bitter, and make up spells like 'Sectumsempra' in his spare time...
While Lily thrived at Potions and bickered with James, Severus was getting into some seriously heavy stuff. Surrounded by friends fascinated by the Dark Arts, he became more and more involved in the wrong side of magic and fascinated by Voldemort's teachings, so much so that he called the one good influence in his life a 'Mudblood'.
Yes, he had been humiliated by James Potter and wasn't thinking, and yes he was suspicious that Lily liked James, but this unforgivable slur forced Lily to end their friendship once and for all. And thus, Lily and James started going out, and the rest is history.
Should Snape have never been sorted into Slytherin, he wouldn't have spent nearly as much time with the wrong sort of people, he wouldn't have received James and Sirius' disdain, and his friendship with Lily wouldn't have abruptly ended. Lily's good influence on Snape was now lost. Maybe Lily and James wouldn't have got together. Think, if Harry had been sorted into Slytherin, he most likely wouldn't have been friends with Ron and Hermione. What then? Well, we pondered that scenario here, actually.
But the domino effect of Snape's placement doesn't change everything. If Snape hadn't been in Slytherin, Voldemort would have still risen to power, yes. But Snape might well have been on the good side from the start, and wouldn't have betrayed Trelawney's prediction to Lord Voldemort. Maybe the first wizarding war would have ended differently.
But for Severus, it was just the beginning of his heartbreaking story arc which saw him badly bullied, make a decision which caused him to lose his one true love, then die while redeeming himself and bringing Voldemort down once and for all.
Who knows what could have been, had the Sorting Hat chosen a different road - or if Snape would have still made too many wrong decisions.