I saw on the Arthur Wiki that there is a poll as to which currently unreleased Season 16 episode will have a guest star/s. Most people voted for "Baseball Blues," probably because they're likely to bring back at least one of the Boston Red Sox as an Elwood City Grebe, or maybe a new Red Sox. The choice with the second-highest amount of votes was "multiple."
"Baseball Blues" would be my first choice. My second choice would be "The Best Day Ever." My third choice would be "So Funny I Forgot to Laugh," which would most likely have a comedian.