Can't see it happening.
I have no insider knowledge, but with two parks widely considered underdeveloped, it makes more sense to concentrate development in those areas. 1 more park means more time on buses and in parking lots, and less time in shops and restaurants. That's less revenue. Granted, it's worthy of a sharp ticket price hike, but that's going to take an absolute fortune to build. Spend too little, and you have a weak park that's going to get ignored because there are better things to do. Spend too much, and you're going to be waiting a really, really long time before you recoup the investment.
Land restraints are quickly becoming an issue, as well. It might seem surprising, but when you sell of property to build Celebration and Golden Oaks, that tends to happen.
Maybe a fifth park can happen in the long run, but it's going to be a ways off. Still, I have a feeling that the WDW theme park market is saturated. There are enough parks. They can fill those parks out, but building another one is just going to cannibalize the other four.
Keep in mind, a new park brings not only the cost of the fifth gate, but also expansions elsewhere.
Let's say they build a cheapy fifth gate, DCA style. No one goes. Waste of money.
If they build a super expensive park like Tokyo DisneySea, what's going to happen is that people are going to drop parks like Animal Kingdom with lower attraction counts. Congratulations, they've just spent a ton of money and attendance increases resort-wide haven't justified it. Granted, all new parks are going to cut into attendance at the other four, but long term, they want to increase resort-wide attendance by a big enough number to justify a massive capital investment.
Now, like I said, I have zero inside information. There might be actual statistics on the inside that show that, yes, a fifth gate would be beneficial to the entire resort. Jim Hill would have a much clearer idea of what's going on on the inside, simply because he knows people and I don't. But rational thinking leads me to believe that a fifth gate is not in the near future, and probably not even in the long term picture.
Maybe I'm just not thinking of the right definition of a fifth gate.