The Count's listed the only appearances from "The Muppet Show" characters on Sesame Street that I know of. However, though Beautiful Day Monster and Rowlf and Kermit were the only ones to make appearances on Sesame Street- there have been at least a couple of references to Miss Piggy on Sesame Street. In one of Kermit's reporter appearances, when he was on Old MacDonald's Farm, Kermit comments to a pig that she looks like someone he knows. In another sketch (and I'm not sure what this one was), there is actually a picture of Miss Piggy (with her 1st. season TMS looks) that can be seen in the background. I don't know if there's been any other references, but those are pretty funny.
I guess if Disney and Sesame Workshop were ever to decide to do it, they could still bring some more characters from The Muppet Show to guest star on Sesame Street.
I would personally love to see a return appearance from Kermit- and that would be plenty. But it might be really cool if the storyline involved him and the TMS gang doing a special show live from Sesame Street or something. Then they would definitely need to bring in Miss Piggy, Gonzo, Fozzie, Rowlf, Scooter, Dr. Teeth, Zoot, Floyd, Janice, Animal, Lips, Dr. Bunsen Honeydew, Beaker, Swedish Chef, Robin, Sam the Eagle, Statler, Waldorf and maybe even Crazy Harry and some of the monsters like Thog, Thig, Big Mama, Behemoth, etc.