I seriously doubt that Miss Piggy will appear on Sesame Street, since she has a bad temper and tends to karate-chop anyone who says something negative about pigs. Her occasional snippets of French might also confuse pre-kindergarten viewers...then again, other Muppets have stranger speech patterns (the Yip-Yip Martians and the Two-Headed Monster come to mind).
Like GonzoLeaper, I would love to see Kermit return to Sesame Street for at least one episode. Better yet, I'd love to see a mini-arc of episodes based on his visit, with a few classic song-clips thrown in: the Bayou Alphabet and the original version of "Bein' Green" should definitely rerun then, if Disney allowed Kermit to appear on Sesame Street that long.