I'm going to rec a story to you now. But, I warn you, once you start reading, you won't be able to stop. And there are 34 good, long chapters.
Flesh and Blood by Tolakasa is...well, awesome is what it is. Buffy is Dean and Sam's mother. She had them about three years ago.
But Sam and Dean are in their 20s, right? So how can they be Buffy's sons if she just gave birth to them three years ago.
Well, for various reasons, they had to be hidden for their own safety, in a place where no one (not even Buffy) would be able to find them. And Willow, clever and powerful witch that she is, thought, "Hey, who would think to look for them in the past?" Oh, and thanks to the wackiness of magic, one of the twins ended up physically younger than the other.
Well, in this story, adult Sam and Dean cross paths with Buffy.
If that doesn't whet your appetite, I don't know what will. And, even though it's a WIP, this fic is worth reading now.
So go. Read. Go on. What are you waiting for?