It's a matter of how much revenue you can secure of the average Orlando visitor. I don't think Disney cares to add a 5th gate, if they can add content to their other per-existing gates. Turn a park that holds your attention for one-day into a two-day park. I think this is why you see the emphasis on Disney Springs, and the added content to AK and DHS.
Incentivize vacationers to stay on DISNEY property. Make it easy to not need/want a car. You then force people to choose between the companies.
If Disney can add content, and do it in a way to conserve labor costs, adding guest capacity, then it's a no brainer. I don't think Disney suffers from the lack of design ideas, more the lack of space in the correct spots.
Putting it another way. It would cost a lot more to add a new 4000 acre gate, than adding 1000 acres to each of MK, DHS, AK and EP. Not only would it cost more to construct up front, it would cost more to maintain. Transportation costs alone would be enough to make some gasp.