We've come a long way since Vanilla, power-wise, and you have to wonder how much further it can go. If every new expansion means higher character levels and higher circles of power, how much longer till we have to call it enough and move on to a new game?
What would be interesting is if they found a way to create exciting new content at lower levels. How to do this? I mean, it would have to be legitimately fun to play, not feel like we were going backwards, but somehow make it feel new and exciting again to play at lower levels.
I suppose they could just start a new game, but what if... they just began a new continent (like Pandaria) but this time make it 1-60? With new quest zones, storylines, instances, even some new classes? To keep us from just switching to mains, they say those characters haven't discovered this side of the world yet, while leaving it open for that to happen at a later date.