In His spare moments, Severus Snape reflects upon what his life could have been.
Today he thinks about Lily, which really isn't saying much, because somehow his thoughts always go to Lily. He thinks of his life with her, he thinks about his love for her, something she will never know because she is, of course, living with the dead. Thinking of Lily brings him to James, and thinking of James causes Snape's mouth to turn bitter.
So, it is on the evening, while the rest of the castle is sleeping, Snape thinks of Lily. With a swish of his too long cloak (he never had proper fitting clothes in his youth, so why start now) he paces quietly around his office, its dreary décor causing Snape to think that is appropriate when thinking of Potter.
Stupid James, him and his wild hair, perfect nose and charming ways. Stupid pranks, making him the laughing stock of the school. Stupid Hogwarts, making him a fool. But here he was, murderer of Albus Dumbledore, Headmaster of a school now devoted to the dark arts. Her he was, successful, while James Potter was dead.
Snape stops and frowns because, no, James was never entirely dead. And he never will be (unless, of course, that silly boy fails, which Snape hopes secretly he won't). A part of James Potter lives inside that brat, and the stupid part is that young Harry Potter knows it.
It is at this point that Snape imagines what Harry Potter would look like if it had been himself who married Lily. For one thing, Harry would not have a scar upon his forehead, because the prophecy would not exist. Snape thinks that it is likely that Harry Potter would have gotten his greasy black hair and too long nose. Perhaps he would even have Snape's too pale skin. But Snape knows that no matter what, Harry Potter would have gotten those beautiful emerald orbs of Lily's.
Snape thinks he would have been a good father. Well, not exactly good, but better than his own. Snape would have actually cared about Harry, he would have ensured tat the boy would never have to associate with common muggle (or perhaps not, because Snape is sure Lily would have protested). Snape thinks that Lily would have been the perfect mother to their imaginary son, but then Snape reminds himself that this imaginary son already exists within another real one.
Snape wanders over to his desk, lost in his thought. He doesn't realize what he is doing until he has already done it.
He had taken out a picture of himself, and taped next to him in the picture was Lily Potter, and below them, sat a young Harry Potter with long black hair coloured on in black crayon.
A single tear rolls down Severus Snape's face before he quietly puts the picture back and continues to think about what his life could have been.