Oh some excellent suggestions there.
Personally, I would LOVE to see Martin Wood come so will third the above. I think he's an awesome director and his commentaries are always so detailed (without being boring) so I'm pretty sure he'd be great on stage. Anyone who saw Tom McBeath's talk at 7.8 will be able to testify to how nice it was to have such insight in a more serious approach. Yeah, ask Martin Please!
As a Firefly fan I'd love to see Jewel Staite and / or Adam Baldwin too.
And Robert Carlyle... just because he looks like my brother and a photo op with him would be a great wind up for bro.