This discussion begs the question: how big is TOO big? There is an invisible ceiling on how much time and money people are willing to spend at Disney World, and at present, you already need more than a week there to really see and do all of the MAJOR stuff, let alone all of the other extras around the resort. As much as we would all love new parks, resorts, and water parks, I don't want to see Disney over-expand, like they did with the Disney Stores, and become too large to even be able to keep all parks open every day. Again, there is an invisible ceiling on how far they can expand, one which I think they are getting rather close to as we see, as Steve said, Animal Kingdom not attracting many new guests, but only leeching guests away from the other three Theme Parks. I, personally, am very pleased with all of the offerings at Walt Disney World right now, and would rather see some work going into the existing parks, as others have mentioned. After riding Mission: SPACE, it is clear there is MUCH which can be done for the existing properties. I think more than a new waterpark, I'd like to see our existing water parks expanded upon, and they could do the same with the existing Resorts--there isn't a big need for completely new resorts right now, but maybe some additions to the current ones might be in order. As opposed to seeing a new Theme park with four or five awesome E-Rides, I'd rather see that money used to add a fantastic new E-Ride and a couple lower class rides (C or D) to each of the parks--and that seems to be the route Disney is taking.
Disney is thinking of building a new Disney Theme park in 2010 or 2009 etc. They will be building Monorails everywhere in 2006 (rumors) and they will be building a new Disney cruise line, A New Disney resort, and finally a new Disney water park. 
