The only two other spots possible are, between Morocco and France, or Mexico and Norway
Even after the addition of the International Gateway, which spoiled 1 (or 2) potential pavilion spot(s), there are many spots left for countries to be added to Epcot. The designers are just very good at disguising the empty spots
Between Canada and UK, are potentially 2 spots, despite the addition of the Millenium celebration building. Actually plenty of room if they take the pavilion up an extra floor or 2, providing great (restaurant) viewing of Reflections of Earth and the World Showcase.
Between France and Morocco are room for at least 2 countries (or, of course 1 double-sized country).
Another 2 pavilions could fit in between Morroco and Japan.
1 pavilion could fit between America and Italy.
at least 1 pavilion could fit between Germany and Italy.
at least 1 pavilion could fit between the Outpost and Germany.
and of course the Outpost could be converted into a full pavilion.
and finally there's room for 1 more between Mexico and Norway.
That's room for 10 more countries.
There's also ways they could represent even more countries with a few tricks...
they could create a little continent where they take one of the double wide sections, let the back section represent several related countries (like South America, with Brazil being the main host), then build out 2 more on the lagoon side (like a couple of the island nations: Porta Rico, Jamaca), each would share a central outside dining area and a stage making the most of the room. The 4 or more countries would keep the stage busy, and guests could choose between 4 or more different cuisines at one location.
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