All of disneys parks have an overall theme. Animal kingdom animals. MK fantasy and adventure, DHS Classic movies, Epcot worlds fair. A new park would have to have an overall theme. They would not build a park based on a couple movies. They could just put those movies into DHS. Star Wars is likely to be put there. Iger says they are waiting because they are going to incorporate the new movies into any Star Wars expansion they do. Expansion of all parks is needed.
The theme concept has been officially eradicated (and gradually going down hill in the years prior to) with the addition of Frozen to World Showcase. I state this not to resurrect the debate about whether or not it deserves to go there, but rather point out that moving forward any addition/expansion will go where it can most easily fit - meaning the least amount of capital investment to get the product up and running. This also more or less defeats the need (not that there ever really was a need) to create a fifth 'theme' park. They've got four plots of land that they can 'plug and play' with for decades to come.