The Muppet Movie guest stars list includes:
Edgar Bergen, Milton Berle, Mel Brooks, James Coburn, Dom DeLuise, Charles Durning, Elliott Gould, Bob Hope, Madeline Kahn, Carol Kane, Cloris Leachman, Steve Martin, Austin Pendleton, Richard Pryor, Telly Savalas, Paul Williams and Orson Welles among others and it didn't turn into "their movie" so I'd have to disagree as long as they're kept as cameo roles as has already been mentioned.
I just see too much undue concern. All this great news is getting skewed and picked-apart. Segel appears to be making this Muppet film the way the classics were. I don't think a fan can ask for anything more. I don't think anything more can be done.