According to the "Keys" park tour guide I had a few years ago, of all the land Walt bought for WDW, only about 40% has been developed so far. They will only develop a total of 80%, leaving 20% untouched. So that means that if Disney want's, they could double the existing WDW. That means up to 4 more parks, two more water parks and double the hotels!
Of course that may never happen, but they do have the space!
I think their next park will have to compete with Universal IOA, i.e. lot's of big thrill mega roller costers. It's the one thing Disney hasn't really done yet. It would also lend itself to a Villian themed park quite nicely, I think!
Whatever Disney has in the works, I'm sure we'll all find out soon enough!