Well, at the beginning of this thread, the OP had a map showing all the areas still deemed "suitable." His blue arrow points to the spot that, by all accounts, is the most perfect for a 5th Gate. Observe:
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As has also been discussed, the green "unsuitable" areas have precedent for being usable for parking lot space. Thus, it's an ideal "easy" area for a park, and a resort in the adjacent "red" to the right of it, just south of Ft Wilderness.
Now, as you can see, there's another "big red" RIGHT next to Wide World Of Sports (bottom of WDW, middle of the map...but for obvious reasons, that's a little busy. And the "red" right above the Animal Kingdom reserve (Kilimanjaro Safaris) would require a lot of finesse--the animals PROBABLY wouldn't care for the noise.
Other "possible but less likely" areas include the potential "butterfly" with Disney Hollywood studios...
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...and the corner between Magic Kingdom and the golf course.
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Again, both would require some finesse. The last has the most POTENTIAL of all the possibilities...but achieving that potential (fixing up all the yellow "marginally suitable" parts of it) would require a lot of work.