Muppets: Almost any character who wasn't retired for being "dated"; Mumford and Herry are my particular favorites, with Little Bird coming in a distant third place.
Humans: Linda and Olivia both left the show because their actresses got better jobs, but there was no "in-story" explanation for their leaving the Street. I'd looove to see them return, if at all possible...
Recurring sketches: Monsterpiece Theatre; if the original reference feels dated now, why not turn "American Monster Classics" into a new equivalent?