I was thinking of an idea today, though it could be a little weird.... Basically, the Muppets go on a trip by airplane, but Gonzo goes onto the wrong plane, not realising it's the wrong airplane until after it's taken off, so Gonzo must jump from plane-to-plane to catch up with the right plane, occasionally boarding big airships (like the ones in Super Mario Bros. 3 and The New Super Mario Bros. Wii). Other possibilities would be that Kermit couldn't afford to bring everyone on the plane they were traveling on, so Gonzo volunteered to ride a much cheaper plane, intending on jumping from plane-to-plane in the sky as his newest act. Or perhaps it could be Sweetums going from plane-to-plane.
Or maybe an idea where the Muppets must do some kind of task, and players can pick from perhaps ten different characters (right now I'm thinking Kermit, Piggy, Fozzie, Gonzo, Walter, Animal, Rowlf, Scooter, Pepe, and Rizzo), with various other characters providing weapons for temporary use (Bunsen and Beaker provide gagdets from Muppet Labs, Beauregard provides tools, Lew Zealand provides boomerang fish, Crazy Harry provides exploding devices, The Swedish Chef cooks up some food to use as weapons, Sweetums provides large clubs, etc.) and perhaps certain characters could help at certain times.
Or maybe a game made up of five or more stand-alone worlds of four levels each, each featuring different playable characters and unrelated tasks. I think somebody mentioned something like this here already. Some worlds could resemble episodes of The Muppet Show, others could be more original.