If Vol 3 does include full episodes, it's probably safe to assume that they'll try and pick guest stars who have a wide following so they can sell more copies. I'm sure the 'Star Wars' episode was included on Vol 1 because it would sell to Star Wars completists (a bigger group, I suspect, than Muppets completists, though obviously there's some overlap via Yoda). Similarly, Roger Moore on Vol. 2 appeals to Bond fanatics.
From that point of view, Julie Andrews and John Cleese could be good choices (to catch Sound Of Music and Python buffs).
And a thought on episode editing: while including full episodes eliminates the need to pay for editing, it ratchets up the cost of paying for music licensing -- which can be a major problem for DVDs (and even videos, as Muppet Family Christmas demonstrates). The preponderance of Pigs In Space and Vets Hospital on the first two volumes might be because it's cheaper than paying the costs required to clear certain songs.
As for the Fraggles, what would be really cool would be to use DVD branching technologies so that the Workshop scenes from all the international editions were included, and you could choose which versions you wanted. Unlikely to happen though, as again artist clearance costs would go up and most people probably just want to view the version they grew up with!
Cheers
Gus