I personally hope we don't see a parking structure at WDW because it would be very difficult to hide it from view as well as you can hide a parking lot.
Also, if you have never been to the parking structure for Disneyland, let me tell you it has a ton of problems. First, there are only a handful of elevators and one enormous escalator. For anyone with a stroller that doesn't want to risk their child on the escalator, you have to take an elevator. The lines for the elevators get very long, as the elevators are not much bigger than average size. When there are only 2-3 operating, the lines get even longer.
Then, when you get to the ground level, you still take a tram to the park, except in this case instead of having multiple tram stops all over the parking lot, you have just one, and again, the lines get VERY long. Especially for those few rows designated for handicapped accessibility. So imagine waiting out in the hot sun for your turn to get on the tram. I know you already do this while waiting for a tram in the parking lot, but now imagine waiting for several turns instead of one or two. And along with these long lines come a greater percentage of people who are less than honorable about waiting their turn. In my opinion, this system is a huge mistake.