I don't think they could do that, as some bosses are gear checks rather than skill, so even if mechanically they can be compared to a current mob, numbers might not let you to kill it. There is plenty to be entertained at, until you clear the last raid, so I would say, when nax comes out and you clear it, thats pretty much it then. But that is what we do in retail, we kill last boss, get CE and tone down to one or two raids a week and farm form mounts until next tier. Pretty boring, but it's intensive 2-3moonths progress and then you can just chill for 2-3 months ez.
Anyway, what I meant to say, you can create your entertainment, if you want to.