Bruno,the infamous heavy in the season 5 episode that guest starred Shirley Bassey.(Pity Miss Piggy was absent from that episode.Bruno would have deserved a "HYAAH".)
I think he had brief appaerances in Roger Moore's episode,as well as Paul Simon's episode.But he seemed to play a different role each time.He's never been seen since.
I miss Hilda.They should have kept her on the series beyond season 1.It would have been interesting to see how she would have related to later characters like Bouragard,Link,Strangepork,Annie Sue,or Pops.Not to mention the makeover versions of Fozzie and Miss Piggy(as opposed to their season 1 prototype versions).
Nigel.He hosted the original Muppet Show pilot"Sex And Violence".But then his role was reduced to orchestra pit conducter in the first two seasons.By season 3 he was long gone.
Wally.I could be wrong,but,his only appearance was host of"The Muppets Valentines Show".