I thought I remembered the author for this fic, but I couldn't find it when I searched her masterlist.
It's Dean/Castiel, but the thing is, it's written in reverse chronological order - that is, it starts with the end and ends with the beginning. The premise of the fic is tragic - Castiel, after hundreds/thousands/millions of years, is alone in the world, living life as an immortal fallen angel.
Certain details stick in my mind: Dean died and went to heaven, where there is a version of Castiel and both are living out paradise. The real Cas, trapped on Earth, knows this but doesn't want to try to contact Dean because he knows Dean would spend the rest of eternity trying to get to Cas. Cas would rather have Dean believe the Cas with him in heaven is the real deal, than ruin Dean's paradise. Other details: the Impala in a museum, Chuck's books.
I would really appreciate it if this was found. This is one of those epic fics that leave a deep imprint in my mind, and now that I've suddenly thought of it, I need to get it out of my system.
EDIT: FOUND - http://tracy-loo-who.livejournal.com/90461.html