What were in my childhood the first-tier Muppets (Big Bird, Oscar, Bert, Ernie, Grover, Cookie Monster, and to a lesser degree Telly, Elmo, Snuffy, and the Count) are all still with the show (save Kermit, now separately owned). So an appearance by the then-second-tier Muppets (Herry, Guy Smiley, Mumford, Forgetful Jones, Alice, etc.) would be nice.
But that pales in comparison to the expectation of the longer-running human cast members showing back up (for the special at least, if not on the regular episodes).