Wasn't Walt's original plan to have 5 or 7 resorts around the lake? (Of course they were supposed to be themed after cultures like the Poly, but then they built the Contemporary and GF)
Walt's original plan was to build an Experimental Prototype Community Of Tomorrow on his Florida land. But when he died in 1966, the plans largely died with him.
Walt's influence on Walt Disney World--which opened nearly 5 years later--was minimal. Left with thousands of acres of land and no guiding vision from Walt, designers basically turned it into a large-scale clone of
Disneyland.
I read that there was/is a soil issue with some of the other proposed sites for other resorts
Correct. Disney had looked at building another resort between the TTC and Contemporary, but found the land to be unsuitable. Walt Disney World is built on a swamp, so there are limits to what they can accomplish. Whether that will continue to be the case as building technologies evolve remains to be seen.
Adding DVC to the Grand Floridian and Poly makes the most sense in the immediate future. I agree with spiceycat that the GF seems the next logical candidate since Four Seasons is building a five-star hotel on-site.
But I wouldn't discount the possibility of another Contemporary tower, either. I refuse to believe that Disney's long term plans involve keeping the South Garden Wing intact next to the original CR tower and the BLT.
Beyond that, hard telling what direction Disney may go. I think they will always be protective of the monorail line. They don't want to dilute that experience for paying guests by overcrowding. And since DVC owners pay the transportation budget, saving money really isn't an issue.