I really enjoy Typhoon Lagoon, It's one of the best water parks I've ever been to from an ambiance standpoint. The Lazy River, Shark Reef, and The Wave Pool are all top notch.
That being said, Water Park technology has jumped dramatically in the last 5-8 years, and Disney has fallen behind. We are kind of coming in to a waning period of a water-park boom across the country. There has been, for lack of a better phrase, a butt-load of regional water park expansions across the country (and in some cases world-wide). That also being said, I really don't think it is, or should be a priority right now. The four major parks here at the Walt Disney World Resort are ALL in desperate need of rehabs/expansions. Maybe once Fantasyland, DAK expansion (whether Avatar or Otherwise), new ride at Studios, and a few more re-imaginings at EPCOT, maybe then we can explore the option of changing/restoring the water parks.
Americans are looking for more and more new ways to keep cool, and seeing the new and exciting things happening at Regional Water parks, people will begin to expect bigger and better things from Disney. But the water parks have never been the spotlight. My point is that they will need expansion eventually, but I think we can put that on the back-burner...For Now...