I have honestly never understood why people would want, or think Disney would want, to build an entirely new 5th gate. Leaving aside costs, what does a 5th gate get you that expanded offerings at the existing 4 parks wouldn't. The only 2 reasons for any business to expand with new offerings would be either to 1) increase customer base or 2) increase spending of your existing customer base.
As far as 2, I can only speak for myself but I only have the ability to take off a certain amount of time per year, and a limited amount of time each time I do take time off. I don't think most people are taking 2 week/3 week vacations. Are people really going to extend their Disney trips even further, with more hotel costs ect., or just go to the new park, and not go to their least favorite one?
As to 1, and i will admit I changed my mind slightly as tying this, I could find reason for building a 5th gate, in a very limited circumstance. In my mind even with some cross, each existing park has something unique about. After MK, Epcot was about the future, AK about animals, HS originally about studios life, old Hollywood, and now I look at it as the main IP focused movie park. Perhaps a 5th gate purpose built around an older/higher intensity demographic? Opens later, higher thrill rides, later night offerings like cocktail parties ect. Its still Disney but maybe you would get more people that feel they age out?