I heard that Bill Prady said that the show can have any possible Muppet character appear. I guess he means any Muppet character that Disney owns the rights to, but I wonder if he also means any obscure character whose puppet is still useable, or if they will rebuild any obscure characters if the need arises.
Suddenly I feel like it would be great if there was an official list of all Muppet characters whose puppets are in good enough shape to be used. Judging from the last two movies, it seems like most of the Muppets Tonight/Muppet Treasure Island-era puppets are still in good enough condition (though there are quite a few who we haven't seen since that era, like Bill the Bubble Guy, Seymour, A. Ligator, Gary Cahuena, Snokey Bloir...).
I think the only minor characters from the Simpsons not to have their own episodes are Larry and Sam (the barflies seen in the background at Moe's), Leopold, Apu's brother, and Squeaky-Voiced Teen.