AU (split at Afterlife) future. Spike = rock star. Xander = manager. I know, I know, but it works. It manages to work with canon, even. As well as being funny, charming, and somehow making a tequila shot party with Willow, Dawn, Spike and Xander seem completely believeable.
(I love mcee's Here's Your Heinecken for not dissimilar reasons, but that story doesn't pretend to be anything other than utterly AU, no backstory whatsoever.)