The Horseshoe Nails series is a Firefly fanfic (Jayne/River) that attempts to rewrite the entire Firefly canon from the end of the series (Objects in Space, specifically), through Serenity and beyond. It does this by focussing on the unusual relationship between two of the main cast (as listed above), and it does this so well as to be one of the most flawlessly written fanfic’s of anything I have ever read.
In fact, it does this so well this is literally the only thing (published book or fanfic) I have read three times in a row back to back without stopping.
The series is broken up into five individual stories, the first three taking place before Serenity, then the fourth being a 22 chapter whopping replacement of Serenity based on the previous changes to canon (and yes, it is a heart stopping adventure of mayhem and wonderful madness), and then the fifth being post-Serenity awesome.
As far as I’m concerned, this is now my head canon, so I share it all with you.