For all the people who think a fifth gate would just take away from the other four...it would a bit.
I think so many people look at Amimal Kingdom as doing just that, but did it really? If you look at park numbers right now, the four parks combined brought in around 45 million guests. We're MK, Epcot, and MGM bringing in that many people before DAK? Nope. More like 35-38 million. Yes it took away a bit from Epcot and Studios, but not severely.
So often people see it as unnecessary just because the current parks are all in need of help. I agree. Build them up and then consider a fifth gate.
Realistically, if Disney announced a fifth gate tomorrow, you can guarantee people would come if it looks good. Heck, I'd most likely try to be there opening day!