Each park needs to have a flow of new things coming to excite the guest to return and see what they missed. If the parks stagger the attractions / Rides then there is always something new for next and a lot of new every five years. At the rate Disney builds the costs to do this a pretty small, but the rewards are great.
Disney is in a much better place today than ten years ago, however they now have serious competition they didn't have 15 years ago.
I suspect you will see a small attraction / ride, and or a big attraction every year in at least 1 of the 3 of the parks.
The magic Kingdom will continue to need expansion because everyone wants to go there. These expansions will take longer and probably involve a people eater or two in addition to any E ticket type rides. Picture an Adventureland or Frontierland expansion similar in scope to the new Fantasyland both in time and costs about every 5-7 years and these expansions should run separate from the work done at the other three parks.
As for a fifth gate I guarantee it would be on the way already if they had the park rights in Florida to the Marvel IP "A" level characters.
Disney could easily fill half a park or more with themed lands featuring Wakanda, Latveria, Atlantis, Asguard, Attillian, and more.
A fifth gate means more hotels, restaurants, and time shares to sell.
Over the next 20 or so years I see 2 to 3 major MK expansions with about 4 big attraction and 4 small attractions for each of the other 3 parks and a fifth gate
The other parks will become MK 2, MK3, and MK4