While we're on the whole M*A*S*H kick, I would've liked to have seen McLean Stevenson or Jamie Farr on the show.
If anyone hasn't mentioned Walter Matthau, he would have been great with the Muppets. I could see him interacting quite well with Statler & Waldorf. I also wouldn't have minded seeing Buddy Hackett, Hermione Gingold (if you don't know who her, she was Gigi's grandma in "Gigi" and the mayor's wife in "The Music Man"), John Travolta, Gabe Kaplan (he and Fozzie would've gotten along great), Carl Reiner, Henny Youngman, James Caan (who is actually a half-decent singer), Flip Wilson, Carroll O'Connor, Bea Arthur....
The list is endless.