This is my interpretation of how an interaction between Snape and the Potter's would've gone a few years after Lily and James had left Hogwarts. (Even if Voldemort didn't exist I still think the confrontation would've happened where Snape called Lily a mudblood and she still would have realised he wasn't the type of person she wanted in her life)

Severus Snape wasn't paying much attention to his surroundings as he left Knockturn alley to appear on the sunlit streets of diagon alley, which is why at first he didn't take much notice of the family walking ahead of him, until he heard a voice that sounded familiar. Looking up in shock, he saw that he was right, a little ahead of him was Lily Evans, his former best friend who he hadnt spoken to in years. Her voice had changed only slightly, becoming a little more mature, but he still recognised it. For a moment, his heart began to beat faster, his breathing uneven as a rush of joy swept over him at the sight of her. That was until he took in the full picture and saw non other than James Potter walking beside her, and even worse, they were both holding the hands of a little boy with Potter's messy hair, who was toddling between them.

Severus had heard she had married Potter, and he'd also heard they'd had a son, but this was the first time he'd seen her since getting that news, and the site sickened him.

Still, he couldn't help taking her in, her red hair was glossy and shining, her steps light, and though he couldn't see her face, when she let out a loud, happy laugh at something Potter said, he imagined her green eyes were twinkling. This never should have happened, this reality were she was married to Potter, where she had a child with him. The sight of it brought back the hatred Severus had for Potter ten-fold. And despite himself, Severus knew he was still weak for Lily, because when her and Potter stopped ahead of him, having a quick conversation before Lily kissed him, then took the child into Florish and Blotts while Potter headed off in another direction, Severus couldn't resist the temptation to follow her.

So far Lily Potter was having a great day. Her one-year old son had finally started to sleep in, allowing her to have a lazy morning in bed with her husband, then they'd had a nice breakfast together before coming to London to do some errands. They'd spent the day so far walking around Diagon Alley, her and James enjoying Harry's reaction to everything he saw, his excited giggles as magical to her as anything surrounding them. Her and James had just decided to split up so that he could pick up some more floo powder (a safer mode of travel while Harry was still young and until the second baby was born) while she ran into Florish and Blotts to have a look for some new bedtime stories for Harry.

She picked Harry up as she came to the children's section and started looking at the shelves.

"See anything you like, Harry?" she asked him.

"Tha," he said, pointing his little hand at a colourful book that had a quidditch player on the front. Lily laughed. "Of course," she said to him, "your just like Daddy, aren't you?"

She picked up the book and was looking at some of the other ones when a voice behind her made her freeze.

"Lily," he said, causing her to whirl around.

Severus watched as she turned to him, he saw the shock on her face, noticed the way her arms tightened slightly on her son, as if protectively, took in the healthy glow of her skin, the permanent sparkle in her eyes, and the roundness of her stomach that told him she was carrying another child.

He waited for her expression to change, for either the same delight he felt at being reunited with her to be reflected back at him, or for the hurt and disgust she'd worn when they'd last spoken so long ago to reappear. What he hadn't expected, was nothing. Her expression became blank, as if he were just a stranger. There was no happiness, but also no displeasure at seeing him.

"Snape," she greeted, neither friendly nor rudely. Snape, not Severus. And that's when he understood; there was no expression on her face because he no longer meant anything to her. She didnt miss him or hate him, she wasn't glad to see him or annoyed, she really didn't care. His presence really didn't matter to her either way. She'd completely moved on to the point that Severus' existence was no longer relevant to her life.

It was so much worse, he realised. As she looked at him calmly, with distant politeness, he wished she would look hurt or disgustsd or angry. At least then she'd still care, and that was so much better than not mattering.

"You married him," Severus finally said. Maybe it would have been better for him to just walk away, let the past remain as it was the way she had, but he couldn't, because where Lily had obviously found it so easy to forget she cared about him, he couldn't forget how much he still longed for her.

Her eyes narrowed slightly, but she just nodded. He couldn't help the way his gaze cut to the child. The boy looked like Potter already, but as the child looked at him with an innocent, happy expression, he saw that the boy had Lily's eyes.

"How?" Severus sneered suddenly. He knew he'd regret letting out his anger out later, but he couldn't take it, couldn't take the way she was looking at him like a stranger, couldn't take seeing that child.

"How what?" she asked, calm and cool.

"How could you marry Potter? How could you have a child with him? You were above-"

"Don't," Lily said, eyes flashing with anger, "you dare insult my husband or my son."

He reeled back in shock and the sudden vehemence in her tone. Where moments ago she was almost emotionless at the site of him, the mere mention of Potter and she was as much of a spitfire as she'd been when she was a teenager.

"But Potter-"

"I'm a Potter now," Lily practically snarled.

"He's arrogant-" Severus didn't know why he kept pushing it instead of just walking away, but he was filled with so much anger and jealousy, hurt and bitterness, that he couldn't help it.

"No one stays the same as the person they are when they're fifteen," she said, and she was angry now, but it wasn't because she cared about Severus, it was because she was defending Potter. "James is a good man," she continued, "He's a brilliant husband, a fantastic father. And i would change nothing," she told him, "there is absolutely nothing I would go back and redo, because right now there is nothing in my life I'd want to be different. I'm so, so very happy with James and our family." Lily paused, and for the first time since he'd approached her, her gaze softened slightly. "I am sorry you don't know that happiness Severus, I hope one day you find it, but there's nothing left for either of us to say to each other."

She was right, not because there weren't so many things he wanted to say to her, there were, but he could see that none of them would matter.

Footsteps approached them and then Potter appeared, he glanced briefly before at Severus before going to Lily's side, putting a gentle hand on her shoulder. She looked up at him, and Severus could tell they were having a silent conversation, she smiled warmly at him, nodded, he nodded back.

Potter met Severus' gaze and Severus was once again prepared for hatred, he had a retort ready on his tongue for whatever childish insult Potter was about to hurl at him, but just like with Lily, Potter's gaze was indifferent. He didn't care about Severus at all, passed thr point of making sure Lily hadn't been upset by him.

"Goodbye, Severus," Lily said, and they both turned around and walked away. That was it. No confrontation, no argument. It fully hit him then, that neither of them cared at all that he was there. They had so completely moved on with their lives, completely left that Hogwarts rivalry in the past that seeing them him had meant nothing to them. He wished he could too.

They were still leisurely browing the books in the store and as he left Florish and Blotts he overhead one last snippet of their conversation; "How much of that did you hear?" Lily asked Potter. "All of it," Potter replied casually, "I realised as soon as you left that your purse was still in Harry's day bag, so I came after you. When I saw him approach you I decided to give you some privacy to talk to him in case you wanted it. Are you okay?"

"Yeah, James," Lily said happily, "I really am good. And I really love you."

Severus could hear the grin in Potter's voice as he replied "I love you too, Lily."

And that was enough for him, unable to take any more, he left the bookshop without looking back.