I dunno, I think it was probably reading yet another discussion about who the "real" Slayer was, and whether Buffy's death would cause a third Slayer to be called. From there it was just a short jump into my AU.

One scene that I really wanted to include would have had Buffy!Dana meeting up with Willow and Dawn - just before the third flashback - but in the end, it seemed a bit unnecessary, especially given Willow's reaction at the end.
Plus, getting rid of that scene allowed me to keep the whole thing much more creepy, with a lot more doubt about who's really in control...