We interrupt your discussion with my random thoughts. Harry Potter spoiler alert!
I think that, had Snape been born into a different family, he would have been a Gryffindor.
Then he might never have met Lily, but he wouldn't have been a death eater to hear Trelawney's prophecy 'bout Harry, or maybe another death eater would have been doing the business Snape was and then he would have heard it and then Lily would have never been able to save Harry and Voldemort might not have been defeated.
Or Voldy would still be here and Harry would have both parents because Voldy wasn't exposed.
Did you know, and this gets me every time, that when Voldy goes back in his memories in Deathly Hallows he thinks about how he could have forced Lily aside? Because Lily had already pretty much given her life after offered the chance to live, the protection was already sealed; if he would have just pushed her aside Harry still would have lived!
My philosophies of the day.