I don't have any fantastic advice but I just wanted to say: I hear you! I've been working on a post-S3 fic for the past 6 months... I think I've been stalling partly because as I got through it I started to realize that I might be setting myself up for a world of crappy comments if I post it. And then I started to feel so much pressure to write, as you said, a "definitive" version. So I have to keep telling myself - this isn't the definitive version of anything, it's just an exploration of one possible reality.

Anyway, I know just how you feel and I can't wait to read what you write. And I hope you enjoy writing it :)