this has been discussed...
I believe that for along time Disney was looking to buy a large amount of land in Texas, probably for the rumored America park. As far as I know they never actually bought it.
in CA, they have already announced where they plan on building the third park, although whether it will open this decade is up for discussion.
I think WDW has virtually endless expansion left, they really don't need to go look for other building sites unless they want to. I do believe that further expansion will necessitate completion of an extended transportation system, because buses to/from 5 parks and 3 water parks, etc. etc. would be out of control!