This all assumes they'd be themselves & honest.
John Paul Jones - was he really going to leave Zep in the mid 70s? What was it like to be jerked around by your old band mates? What really happened on the road in the old days?
George Carlin - just because.
BB King - because John Lee Hooker is dead. Just tell me everything about the old days, and play Lucille for me.
Derek Jeter - just to watch the hot girls he casts off, LOL. (Or maybe Tony Gwynn, just to talk baseball all day)
I'll think of 50 others later.