I was just thinking about Marc Maron as a guest, too. I've been listening to Marc's WTF podcast, which has been a real mix of comedy, philosophy, politics. He also has some familiar Best Show guests: Galifinakas, Patton, etc. There is a danger of Marc veering off of Hard G, but he had a terrestrial show, so he knows the deal.
I think he would be an insanely good guest, but that's just me.
Pastor Josh recommended Marc's podcast to me, and I have to say it is pretty terrific. I've been listening to it and Best Show nonstop for days.