I remember when the spoilers were leaked for The Gift even before UPN started advertising that BtVS was moving to their channel and spoiled a lot of people, some fans were like, "How can the show continue without Buffy? Can there be a Buffy the Vampire Slayer without Buffy herself?" My thought was yes! That thought, of course, did not come from someone who worked in television, but I thought it could be done. And now that you mention it, there's the plotline for a S6 without Buffy-- Giles realizes that things don't have to be the same, the cycle can be broken. Maybe he talks to the Scoobies, and they try to get to the next Slayer before the Watchers Council. Maybe they start out the season with the same selfishness of replacing/replicating Buffy but then as they get to know the new Slayer, they realize just how wrong they were for doing that. By the end of the season, it sinks in-- Buffy is never coming back, and they have to move on. This is when the reality of grief would hit, and it would be heartbreaking but it would be a glorious payoff. Now, I'm thinking about how awesome that would have been and how Season Six pissed on everything I loved (and the musical episode will never make it worth it).