I don't see a 5th park because it would simply cannibalize the parks they already have open. There isn't enough time in a standard week long American vacation to cover WDW now. Opening a 5th park would just make that worse. There is a huge need to devote the capital budget to maintaining, refreshing, re-theming, and expanding the current parks. Each of which has fading attractions and will draw big crowds to return over and over if they only provided one or two decent reasons on a regular basis.
For example, if WDW would consistently add an attraction and refresh, rebuild or re-theme an attraction, at each park, every 3 or 4 years, they would be in better shape than opening another park. Plus it would give them the capacity to keep up and that would allow them to take attractions down for much needed repair and rebuilding (Space Mountain, Imagination, Indiana Jones Show, and Yeti for example). The staffing needs would only grow slowly, and again I maintain Disney has no problems getting staff, they have huge problems retaining staff due to wage, scheduling, and benefits, and the parks would be in a consistent state of refreshment and improvement.
MK has space for expansion if you push out past the tracks or expand the track loop. EPCOT has acres of space everywhere. DHS is a bit tighter, but if you went behind RnRC you have lots of land, though Cypress Drive might need to be re-routed. And AK might be worse than EPCOT for acres between attractions. So the existing parks are all you need. You just have to invest in attractions, something Disney is just starting to do again after a much too long, and irresponsible, lay off.
I'm not talking about adding a themed, immersive land every 2 years. I'm talking about a new C/D/E type most of the time. Maybe once a decade you add a whole new land in addition to the rest. But continuous investment is the needed option. Not a new gate.
