Possibly the footprint of DHS is just too constricted, all going back to the hastiness with which it was designed and constructed. Now they have to build a multistory parking garage which is bizarre when you think of how much land Disney owns. But they're blocked on three sides by two highways and a wetland.
Now I understand why they're thinking of dropping movie IP into Epcot. DHS is too restricted to be the future expansion pad for "whatever's hot" movie-based attractions. MK and AK are not as thematically appropriate and are both restricted either by railroad and water for MK or by animal habitats/exhibits for AK.
I think Disney will be very hesitant to add a 5th park. Perhaps only if Uni announces a 4th park and Disney really wants to appear to be always in the lead. So short of a 5th park, I think you're going to see a lot of replacement of older, less exciting and more neglected attractions in the Disney parks. The arms race is heating up noticeably. I would be worried if I was a big fan of GMR, Energy Adventure, Imagination, The Land, The Seas, Carousel of Progress, etc.