I agree with more guests from the opposing positions (anti-vaccine proponents, psychics, creationists). First, I think it would be a test for the panel to maintain their composure and, second, would sharpen everyone's understanding of the different perspectives, which is very helpful when discussing a position.
Authors of several recent popular books such as Imagine, Compass of Pleasure, Blink, etc.
Derek Murphy, author of Jesus Potter, Harry Christ.
Jon Stewart - it would be very interesting to get his ideas on how he challenges controversial guests. This may give our community insights in to our dealings with similar guets.
Ginger Campbell from the Brain Science podcast
Deepak Chopra - let's hear from the woo practitioners
As much as I like some of the stalward guests (e.g. Phil Plait, Neil deGrasse Tysen), a broader and more controversial range of guests would deepen our understanding of their perspectives. Sometimes it seems like it's a mutual admiration society of communal back-slapping, rather than a means to challenge our thinking and sharpen our arguments.