Disney has a master plan for their Florida property. The master plan identifies future roads and sites for future resort hotels, future timeshares (DVC), future shopping, and, yes, future theme parks. My understanding is that in addition to the four existing theme parks, there are sites identified for three additional major theme parks.
Having a master plan is a smart idea. It means that Disney won't find themselves having to tear down a hotel to build a future road due to lack of planning.
However, just because there are three additional theme park sites doesn't mean there will be three more theme parks some day or even one more theme park. The master plan is a land use plan, not a long-term development schedule.
At this point, the execs at The Walt Disney Company have decided that there are much better corporate growth opportunities on other continents than at North America's Walt Disney World and that the profits from Walt Disney World should should be invested in other parts of the world and other parts of the corporation (not necessarily involving the Parks & Resorts business segment), rather than reinvested in Walt Disney World.
Disney won't proceed with a fifth theme park at WDW until the execs determine that it's the best investment that Disney can make better than buying cable channels, better than building "
Disneyland" parks in countries with developing economies, and better than buying aircraft and leasing them to airlines.
Things can change. Maybe in five years, a fifth theme park will seem like a great business opportunity to the execs at Disney; or maybe in 20 years; or maybe never. Maybe there will be three more theme parks in 20 years; and maybe not.
Maybe they'll have to redraw the master plan to make room for six more theme parks (there's plenty of land). But I wouldn't count on it.