Any of the folks from shut up and sit down Example
Patrick Rothfuss (guest GM) Example
the other folks from the Critical Role cast, Talesin in particular for me Example
Neil Gaiman (guest GM) Example
Amanda Palmer Example
MC Frontalot Example
Bill Bailey Example
If you do indeed run short of guests, I am probably Available and would most likely be an embarrassment to all parties.
In all honesty I would be happy to just see them rotate through the existing RollPlay Family. Allowing them to run their own game for oneshots as has been done recently with Crendor and Dodger.
Building chemistry between peoples isn't always going to work, and I feel that introducing strangers to eachother live, over a camera, to an audience and then having them play out fantasy characters can make that task even more daunting.
So, the folks above would be interesting to see thrust into a live session or playing with folks they don't usually. But most importantly of all I would like to see people who are comfortable with the other members, up for fun and exploration of the themes set upon them and a confidence in the game they are playing.