As a 'what-if' scenario only:
Disney should consider looking into the Pigeon Forge/Gatlinburg area of East Tennessee. This would be a practical move for several reasons:
1. We don't know what's going to happen with gas prices over the next years/decades to come. The Gat/PF area is only a one day drive from the majority of U.S. residents, and only a few hours from hot population cities such as Atlanta, Nashville, Cincinnati, Charlotte...etc. When gas prices & air fares hit the roof, this area still reels in tourists due to being so easily accessable to the majority of U.S. population. Whereas Orlando & Anaheim sit in far corners of the country forcing out the option of being the pracitcal family road trip.
2. The PF/Gat area is already a thriving, family-oriented area which reels in the types of crowds that would appreciate the Disney theme-parks. Instead of "Build it, & they will come" it is "They're already coming..now build it."
3. Do NOT underestimate Dolly. Many people who have never been to Dollywood would not consider it up to par with the "Big 3" in the theme-park world (Disney, Universal, Busch). However, Dollywood is very much up to that level, (clean, friendly staff, great theming) and head & shoulders above what you would find at a Six Flags, Cedar Fair, or other regional park chain. If Dollywood & Disney were going head-to-head in this area like Disney & Universal are in Orlando, throw in the fact that we are closer to home for most Americans...this area would soon become the new theme-park Mecca for travelers.
Of course, not having an international airport and year-round warm weather would be the hold-backs. But this is just a fantasy idea anyway.
In reality, I only see Disney strengthening DLR & WDW over the years/decades to come. A 5th gate at WDW? Highly unlikely, but not completely ruled out. New theme-park property in another U.S. city? Not going to happen; bank on it.