I believe there used to be plans to build a bunch of suites near where the bus depot is now. Since the depot is huge, and there isn't much land over there, that plan is probably scrapped. When you consider all the room needed for the hotel (building, parking, facilities, landscaping, etc), it's a pretty large project. There's really not much room for a resort in that area.
Epcot really doesn't have a great place to put a resort in. World Showcase has some potential spots for one, but since everything back there is forced perspective, it would destroy the illusion (imagine having a real 7 story building that's standing next to a 7 story building that's supposed to be 10 stories!)
MGM is actually the perfect location for a resort. They could put one right on Sunset Blvd behind the Market. It would also have a great theme that hasn't been used with a hotel yet. Another one could go near Echo Lake, and have an Art Deco theme (again, unused)
AK is the perfect park for a resort, but AKL already exists off the park property. Imagine having a resort near where Everest is now, where rooms would look out over the lagoon and towards Discovery Island and the Tree of Life. Another great resort location would be between Camp Minnie Mickey and Harambe.
Whew