There's plenty of great ideas still available for pinball machines, unfortunately Muppets isn't one of them.
Personally I'd love to see them do a Pulp Fiction pinball, the music alone would sell it to me.
Or if we're going Rock n Roll, how about a Queen pin?
The thing with band pins I reckon is that Metallica looks awesome (and I am not a Metallica music fan) because of the artist behind the work, if it was a pin with a bunch of guys photoshopped on the playfield with long hair it wouldn't be the same. I just don't see too many other band pins with the same art potential, but could be wrong.
I don't think Stern can go wrong with Iron Maiden if they go with the cover style artwork and please the Maiden fans. Seems like there are a few out there and it has the music to back it up with the cool artwork.
I think the Beatles also have a varied catalogue of music and a Yellow Submarine themed machine alone would be awesome.
Cheers!
Adam.