1d20 ⇒ 1
I was thinking do generic Golarion (lots of info out about that world, which lets people have some setting ideas etc), and then each GM can have their own "main" idea for their time. Somewhat like a series of PFS games, Pathfinders get sent all over to do XYZ and sometimes even in the MIDDLE of a current mission get redirected (i.e. GM switch) via sending or whatever.
I also agree with expanding beyond Core Rulebook, but I'm up for whatever.