I think we need to look at Pandora -- the World of Avatar" and see how it compares to the complete picture of a Potterland that will span two parks at Universal and include the areas that are in IoA as well as the expansion that is underway where Jaws used to be, including the train ride that will link them.
As junky as I think the Universal parks are, I think there will be nothing on the planet like the Potter experience at Universal. When the expansion opens, I plan on spending a whole day at Universal's Potterland on future Florida trips. I doubt we'll extend our stay in Florida by an extra day, so one of the days we spent at WDW will in the future be spend at Universal's expanded Potterland because it will be fun to ride the train between the two parks and explore both sides of the Potter world. This is the only thing that would ever get me to set foot in a Universal park and the only thing that would certainly get me to buy a two-park pass to Universal (presuming you need that to see both sides of the Potter attractions).
I think what Universal is building with Potterland is genius and groundbreaking and very exciting.
Can Disney top that with Pandora? No. But I think Pandora will make DAK a full day park for the people who currently don't think it is. I believe the only way to top Universal's Potterland is to completely redo DHS on an even bigger scale than the DCA makeover. I'm really stumped in terms of thinking up an attraction that would be as exciting as riding a real life Hogwarts Express between two parks...so whatever would come to the future of DHS it would have to immerse guests in a movie as well as that. I think the only property that Disney has rights to that could top Potter is Star Wars.
A massive Star Wars expansion of DHS would be the Potter Swatter if guests could feel like they would be spending a day on the different planets of Star Wars.