Personally from a cost standpoint... I can't see the logic in opening a 5th gate right now.
With a new park, you already end up with overhead due to the need to staff the turnstiles, guest services, ticket booths, and the parking lot. You also end up with the need to run additional Disney Transportation routes from each onsite resort to the new park. that's a lot of reoccurring costs before you even start factoring in attractions, dining, and merch within the park.
I honestly believe both AK and MGM (er.. DHS) could use some real love before they open a new park. Currently at AK you have 7 rides (Safari, the train to the Conservation Station, Kali, Everest, Primeval whirl, triceratops spin, Dinosaur), and at MGM you have only 6. (Tower of terror, Rock N Roller Coaster, Toy Story mania, Star Tours, Backstage Tour, Great movie Ride.). There are also several shows in each park (and AK has the walk thru attractions) which help give you more to do... but there is a large subsection of guests who just look at "ride" counts and don't care about shows beyond the nightly fireworks.
If you remove attractions with height limits, the numbers are even worse. AK has only 3 rides (Safari, Train to Conservation Station, Triceratops Spin), and MGM has only 3 (Toy Story mania, Backstage Tour, Great Movie Ride).
So ultimately, bang for the $$... MGM and AK could afford to be enlarged before they start adding another gate. Increasing the attraction count at both parks would go a LONG way towards improving existing wait times. Especially at MGM, If they added some fun attractions that younger/smaller guests could experience, I'm betting we would see a dramatic decrease in the crowds for Toy Story Mania.