What I said last this was brought up is still true today. It is something that I don't see this happening for 5 reasons. WDW has parks that are not a whole day park to to a lot of people. The 2nd issue is cost to run 5 parks. The 3rd reason is Disney doesn't do a lot of maintenance at WDW as a whole and it shows like for Fantasmic!. The 4th reason is people will complain that the 5th gate isn't a whole day park. Walt Disney World World be competing against itself for attendance if there is a 5th park. That means Walt Disney World Attendance wouldn't decrease, but it wouldn't increase at the time. That also means the 5 parks will be fighting for the same person.
While Avatarland is going to turn AK from a half a day park to anywhere from a 3/4 of a day park or a whole day park, DHS needs a lot of work from turning it from being a half a day park based on what a lot of people thing about the park.
DHS has more stuff to do than it did back when I first went there in 1991, but at the same time DHS isn't what you call a whole a day park. DHS to do something with Sounds Dangerous, and Studios Backlot Tour currently in terms of replacing the attractions.
WOW -- we spent 9 days at WDW in September -- did NO water parks and only DTD one afternoon and still we could not get to do everything we wanted to do in each of the parks and it not like we took days off or big chunks out of our day to enjoy the resorts, pools or just sleep in....I think I would explode if there was a 5th gate right now.