Not sure if these actors have been mentioned:
Tim Curry. They could have done a slight nod to RHPS, and with the Muppets, it'd be a real classic!
Billy Crystal. To boast her ego more, he could have told Piggy she rooks mawvolous!
All four of the Beatles, though it was after the band broke up, but I heard Paul McCartney was willing to do it. John would have been great to see as well, or Ringo, or George!
Any surviving principal stars from "The Wizard of Oz" such as Ray Bolger, Jack Haley or Margaret Hamilton. Buddy Ebsen perhaps, too, but, he was more well-known as Jed Clampett, and that could have made for a fun episode.
There's a bunch of others, many of which have been mentioned. The show should have lasted longer, plain and simple!