The fact that he and Lily Evans had never met was not, by far, the largest difference between this world and the one in which he had been born, though it was not so very far away from his own: his father had left his mother, and she had begged her family to take her back. They had refused, though they granted her a stipend, on the condition that she and her misbegotten half-muggle child change their name and move to the continent, distancing the House of Prince from their shame. Severus had attended Durmstrang. His first language was German, and he was far less concerned with the Dark Lord of Britain than his latest treatise on the theoretical basis of mind magic. He had married well, taking the name of his half-veela wife (a whip-smart blonde who found his ability to speak to her without devolving into a drooling puddle of hormones more than adequate compensation for his face), and they were exceedingly content with their two children (nearly grown by the time the other Severus was the same age as he) and a chateau on the Riviera.
Lily, on the other hand…
Lily, somehow, had ended up teaching at Hogwarts.
It was almost as though without him there, she had taken on his fate.
She had been in Gryffindor, and with no reason to hate them, had become friends with the Marauders much sooner. Somehow (Severus refused to look too closely at the details of that particular alliance), she had ended up in a three-way relationship with Potter and Black, though it had not lasted through the end of the war. She had managed to bear twins, a boy and a girl. It was well known that one was a Black and the other a Potter, though the broken triad refused to disclose which child was blood-heir to which House: they were recognized by both, as if they were the product of a Black-Potter alliance, and both answered to the name of Evans.
When the war ended, in the summer of 1982, it was not with a single, legendary event: Dumbledore finally swallowed his pride when the Order of the Phoenix collapsed around his ears, and requested assistance from the ICW. The Death Eaters were also coming apart at the seams by then, their internal coherency fractured by the Dark Lord's increasingly obvious instability. The Peacekeepers had descended upon the beleaguered nation with an efficiency which had not been seen on either side since 1978. They captured well over half of the Death Eaters and the Dark Lord himself. The ringleaders were deported to Nurmengard, to live out their lives like muggles. Many of the Second Circle were sent to Azkaban. Narcissa still managed to save her husband and a few of his closest associates with her well-planned Imperius Defense, though not so many as Severus recalled.
Also unlike his timeline, the Light was not exempt from the ICW War Crimes Tribunals. Dumbledore was ordered to step down from his position in the ICW for the actions he encouraged in the war, but he retained enough power to shield those whom he chose to protect. Lily was one of them. She had, Severus thought, committed the most obvious and egregious crimes on the Light side of the battlefield. The others might have used the Killing Curse in the heat of the moment, and several of their healers had turned to ritual when their charms were not sufficient, but Lily had invoked a goddess and raised the dead on the battlefield. Those events were unchanged, here, or at least similar to the ones Severus had witnessed in person, if not exactly the same. There were those who thought her too dangerous to live, with that kind of power to hand, and more who thought her blatant disregard for the Statute deserved punishment regardless of her reasons.
She threw herself on Dumbledore's mercy, much as Severus had done, and a deal had been negotiated: her loyal service to him, personally, in exchange for his protection. So Dumbledore had stood as guarantor of her actions, and taken her off to Hogwarts. Her children were raised by Potter and Black, while Lily became the general factorum of the Castle in Severus' place. She officially taught an elective in Healing, though her curriculum overlapped heavily with Potions and Charms, and the second term of every year was devoted to 'Preventative Measures' which was, more or less, Defense (though that position still took its yearly sacrifice). Over the course of two years, she worked her way into Dumbledore's good graces (something Severus had never attempted), and convinced him to revive the position of the Disciplinary Agent, which had been vacant since John McKinnon's retirement.
By 1986 she had more or less taken over the leadership of Slytherin House under the pretense of keeping the peace within the school. In 1989, this state of affairs was declared official, as Horace Slughorn finally retired and his replacement was a former Hufflepuff. Lily was unanimously elected as the most Slytherin member of the staff, including the newly-hired Aurora Sinistra, who was actually an alumna. Aurora took over with Discipline, which Severus thought worked out well, and for a time things were stable at Hogwarts.
In 1987, Bellatrix killed the Dark Lord in Nurmengard, freeing his soul. Few knew that a psychotic madwoman had murdered her one-time lord and master, a washed-up, magicless Dark Lord in the depths of Grindelwald's prison. Even fewer cared.
Five years later, just after the tenth anniversary of the end of the war, the Chamber of Secrets was opened. The Evans twins, Hufflepuffs in their second year, were revealed as Parselmouths, raising nasty questions as to where they had gotten that particular talent. Lily Evans smiled mysteriously for the public, and refused to answer them. Behind the walls of Hogwarts, she healed the basilisk and returned it to its one-time mission of protecting the school. The Dark Lord's horcrux she captured intact.
In 1994, the Dark Lord's shade found Barty Crouch Junior, and was re-embodied with the blood of Crouch Senior. Whispers began to spread of his return. Lily offered the horcrux a body and his freedom from the original, for the price of his assistance in destroying their sire once and for all.
Severus suspected legilimency of the highest standard as the only possible explanation for the easy acceptance of Lily's new 'apprentice' in the autumn of 1995 – a wizard of twenty-five years who called himself Todd Oliver Lammor and seemed unable to decide whether he resented his Mistress, or held her in the highest esteem. With only ten years between them in age, rumors abounded that there was more to their relationship than a simple Apprenticeship contract. Severus was, he suspected, the only one outside the duo who knew the truth: they were two bodies and two minds with one life between them, bound every bit as deeply as Potter and the Dark Lord had been.
With half of Tom Riddle's soul as a focus, it was laughably easy for Lily and Todd to track down and destroy the other horcruxes, or most of them, at least. The second war ended as the first had done, with no sacrifice and little fanfare, in the summer of 1996. Voldemort's soul – what was left of it – was drawn into its final anchor, still locked away in the abandoned Vault of Lestrange, imprisoned as surely as he had been in Nurmengard.
Todd took on the Defense Position and broke its curse. Lily continued to teach Healing. The twins graduated and settled into long-term relationships, the boy with Hermione Granger, and the girl, to her fathers' utter horror (and Severus' great amusement), with Draco Malfoy. Dumbledore died in 2002, due to an overnight onset of diabetes, for which Lily reprimanded Todd, and Severus silently congratulated him. Aurora became the Deputy Head to Minerva's Headmistress. The Evans twins held a double wedding two years later. Todd met his match there in N. Tonks Lupin, by then a thirty-four-year-old widow and single mother of three, finally laying to rest the rumors of his and Lily's torrid affair (excepting the ones where the metamorph was simply added into the mix). He left Hogwarts to take up politics when the Democratization bill finally passed, and became the first Commons representative to be elected Minister of Magic in 2010.
With no reason to remain at Hogwarts any longer (her children graduated, Dumbledore dead, and Todd gone), Lily retired from teaching to focus on her research. And without teaching, Severus was disturbed to see, her 'research' grew ever more ethically questionable, delving into the nature of magic, the Powers, and the human mind in search of, as she excitedly explained to Todd, a way to turn muggles and squibs into wizards, so that the Statute of Secrecy might finally be lifted without any danger of muggles hating and fearing that which lay beyond their grasp. Todd, of course, was opposed to the idea of suddenly-more-powerful muggles, and moved to halt what she termed the New Eden Project. Lily fought him, arguing that they would, of course, handle it more delicately than he seemed to think, but insisting that her research was an important first step. Neo-Grindelwaldian Anti-Statutarians flocked to her side, and in 2017, Lily Evans was declared to be a Dark Lady, at war with Magical Britain.