Well, all three sets have significantly different bonus features. The first one had the secodn pilot, the pitch reel, a few promos, and Muppet Morsels on every episode. The second set had another pilot, interviews with the Muppets, and a music video. And this one has two documentaries. It'll be amazing to know what will be on the fourth season set.
But there does seem to be a pattern. Most of the features on the first set were from before the show began. The second set had the other pilot, as the first set had one, and also interviews with the Muppets. The Making of the Muppets probably has interviews (though as of now we don't know if any Muppets will be interviewed in character). And there are two documentaries. I wonder if any other documentaries will be on the next set (crossing fingers for the Jim Henson Retrospective and Of Muppets and Men...). Or maybe the enxt set wil have audio commentary.
I wonder if this new documentary is the "really cool project" that's been announced. If so, then it'd better be a really cool documentary (though many documentaries are really cool).