The expectation that a new park would be built on average every 9-11 years is totally unrealistic. You're either going to run out of land or more likely demand for new parks will level off and you stop building. This has nothing to do with a lack of creative ideas. It's very well known that Animal Kingdom didn't perform as expected in terms of increasing resort visitation and increasing length of stay.I just don't think a 5th park will happen anytime soon. Put it this way, this is how it has gone so far:
Magic Kingdom 1971
Hollywood Studios 1989
Animal Kingdom 1998
There has never been more than 11 years since the opening without a new park. Right now we are at 23 years. If you take away the water parks that were added, that's a lot of time in between. There is space for sure, but it is possible there is also no ideas in order to have a main park.