I can't for the life of me remember the title or Author of this, so:

I found it on a Dean/cas Community. A Fertility Godess or something along those lines decides to do the both of them a favor and grant them a child - a little girl that is half Angel half human. I remember the Fic featuring a lot of other angels, and their reaction to the new Nephilim. I also remember that it was apparently forbidden to create Nephilim and that Dean was kinda not daring to believe the "situation" was permanent, so it took quite some time before they named the little angel. The Fic also included a somewhat wayward Archangel who had, so far, not known anythings about the apocalypse. Gabriel was featured and eventually got together with Sam (I distinctly remember talk about HIS children, including an animal, somehow?) Lucifer was quite interested in Cas and Deans little Girl, as well.

It was collaboration of two authors, and posted in some sort of private writing community of theirs. It was unfinished and I remember from skimming through the journal that they updated it every month or so.

I'd be really really grateful if that rings a bell with anyone!