Talk of a 5th gate comes up here continually. Aint gonna happen anytime soon, especially the period we are entering (economically). Problems also exist with the area surrounding Disney.. 192 and I-4 are already a nightmare, now imagineer all the new employees commuting in for work and all the guests coming to the park on top. Now think about what it will do to WDW infrastructure, more Disney buses and cars bouncing from park-park and to hotels etc.
I am not sure of the stability of that land, I looks a bit swampy, that would cost bigtime to stabilize it for a park, parking lot and access roads. I am sure Disney knows the entire land and feasibility since the 70's.
What would the cost be ? Maybe shoot to keep it at the 10 billion mark (truly a guess based on Epcot being about 1.4 billion 40 years ago)