Cory:
Yes, this topic has been discussed before.
Technically, Pinehurst and Pebble are public, so you have to include them. However, I'd be surprised if the USGA wouldn't be interested in one or two additional municipal courses as options for the Open. However, the list of quality courses already interested in hosting or continuing to host the open is large enough. Additonally, can one imagine the Open in Pittsburgh but not at Oakmont, or in Boston and not at The Country Club?
Given that the Open will likely be held only in major metropolitan areas, the likely areas would be, in my opinion, Philadelphia, Chicago, Cleveland, Denver, and the Pacific NW, either Portland or Seattle.
What sites are possibilities? Not Pumpkin Ridge in Portland or Dubsdread in Chicago, as these are privately owned daily fees, and we are talking about municipals. (Also, these have been "test driven" with US Amateurs, Public Linkses, and Women's Opens and don't seem to have attracted US Open- type interest.)
It is unlikely, also, that the the USGA will front the money for another Bethpage style renovation, as there are few if any muni courses of that quality that could generate such interest from the USGA, the players, fans, etc. (And certainly not at privately held daily fees.)
Consequently, there it seems doubtful that the Open will find its way to more municipal courses. Unless, that is, there were better options than Winged Foot, Baltusrol, Oakmont, Oakland Hills, Southern Hills, Oak Hill, Congressional, etc.