I believe that the TPC courses are under an exclusive contract with EA Sports (Tiger Woods). That takes away about a dozen of the courses used on the tour from consideration.
However as the previously poster mentioned, there are some great courses, that are semi private that would be great. The Lakes course in Auburn, AL is a great tract and has hosted the former Nike tour championship. Colonial and Cherry Hills would probably not agree to be put onto WGT, nor would Southern Hills over in Tulsa.
Pinehurst we get next year. Harbour Town I think we could get and maybe the new Doral after Trump bulldozes the Blue Monster. Any of the 'resort' courses would be an easy pick up, they would be willing to be put on WGT for a small price just to get the publicity.