Besides all the Spuffel fanfic I've read and enjoyed, the immediate inspiration for this vid was an interview with the current Buffy comics writer Christos Gage. Here's the relevant fragment:
If you had your way, and Joss said “yes” to any idea you presented to him, what would you really like to do with these characters that fans would not expect?
CG: Well, this is more in the realm of ending the story altogether, but for me, a true “happily ever after” for these guys involves Buffy, Spike and Angel living together in a polygamous marriage arrangement. Although I’d imagine there are some fans that not only expect that, but have written extensive fanfic about it!
Then, of course, Gage went on and wrote a scene in Buffy S10 #10 in which Harmony (of all people...) fantasizes about this exact same thing; see the image at the end of the credits.
Happy New Year to everybody, and I hope you enjoy the vid!