Canon has lots of great big holes that you could drive a Knight Bus through, but even so, some things that I thought or hoped might happen now seem impossible while other things that I really did NOT want to happen are pretty firmly entrenched. (Most of these have very little to do with Snarry, ironically; my biggest disappointment of book seven is Nymphadora Tonks.)
I think I've written with people who were much stronger than I am in certain areas of writing, plotting in particular; I fail completely at moving a plot from point A to point B most of the time and need someone kicking my butt to make it happen. *g* I think in a couple of cases we've had really different strengths as writers that were very complementary. What's been most important is that we agreed at the outset in general where we were headed; where I've had things fall apart on me collaboratively is where we've had no real plan for the endgame and opposing ideas about what kind of mood/tone we wanted to end with.
My story is only four sentences so far so please don't look forward too much. *veg* Thank you!