Here are the theories by Phinea Rogue and isasniffelus:
This is not really a theory, just some speculation (after exams, it's nice to go speculate a bit...). JKR has updated her site and there she answered to a theory I've never heard before - called "Luna is Snape's daughter". She said, "This is the most tantalising idea, but no, Mr Lovegood, the editor of the 'Quibbler', really is Luna's father and Snape does not have a daughter."
By saying that he doesn't have a daughter, can she mean that he has a son? Why doesn't she say simply that he doesn't have children? Is she just playing a bit with her fans? I suppose that's possible too...
I've never thought of Snape as having a family - as a wife and kids - only possible siblings or parents, but never children. Would he have a son? Who would have married him? Okay, you don't have to be married to have a son, but... you have to have someone to have him with... and although I like Snape a lot, I can't imagine him being in any relationship.
A few of the Hogwarts professors have spouses, but the information is restricted – that much we already know. If Snape had a son, could Voldemort use him against him (if he were to find out that he's no more loyal to him)? But JKR said once that she thinks that the thought of Snape in love is a "very horrible idea" and is stunned that someone wonders if Snape might fall in love. Therefore I've always thought that he's never been in any relationship... then again, you don't need to fall in love to get married...
Narcissa and Snape are very intimite friends it seems. Both were in Slytherin at the same time. She couldn't marry him because Narcissa had to marry a pureblood. Snape's patronus will be of importance. Could it indeed be a flower? What is it that made Dumbledore trust him and that he couldn't tell the staff?
I've thought about this theories quite a lot now, trying to make sense of everything Snape's ever done.
One thing that doesn't stop bothering me is the Unbreakable Vow. In my opinion, there was simply no need for it, and don't tell me Bella would have gone to the Dark Lord otherwise. He went behind Voldemort's back by making the Vow, not by refusing to do so. It was senseless and stupid and uncharacteristically emotional for Snape. An emotional reaction to emotional pressure. I had a discussion about that with a friend and she said it seemed to her like a fatherly reaction... as father's reaction to a mother's despair.
You know, the more I think about it, the more it makes sense to me. And, spinning that idea further, I think a lot of his actions could be explained with that. It might even explain why DD trusted him unconditionally. Consider this: We know that Dumbledore had an iron-clad reason to trust Snape, and it seems to be connected to the Potters' deaths. DD told Harry that it was Snape's greatest regret that he passed on the prophecy and thus made Voldemort go after the Potters, but nobody seems satisfied with that reason (except the SS/LE shippers of course ). Hence I wondered: Could it be that it was Harry he wanted to save, not Lily or James? Just because killing a child was a line he didn't want to cross? Maybe because he knows how it feel like to have a child, or possibly even to lose a child? Then, sixteen years later, he's faced with the same dilemma: His master or an innocent boy. And he makes the same decision again. That would be ironic...he makes the same decision twice, once it makes him a hero, once it makes his evil.
To be honest, that's the theory that makes most sense to me. We don't know if Snape killed DD on Voldemort's order, we don't even know if Voldemort will be pleased with Snape's deed, after all, it was supposed to be Draco's task. Can it be he betrayed both of his masters all at once? For a very personal reason? I wouldn't go so far as to say Draco is his son, but maybe he really doesn't want to be responsible for the death of a child?
Any thoughts?