Looking at the map of World Showcase, I'm wondering where any new pavilions would go - were they to be built. It looks like there's land beside Germany, possibly beside Mexico on the Odyssey. Anywhere else?
There is supposed to be enough room for 6 more countries. I believe there is enough room for 3 more between the bridge where the fireworks barges come out, and Germany. If you look at aerial views of the park, at www.themeparks.com, {go to the bottom of the page and click on photos, then disney, then aerial, you can try to figure it out for yourself as to how many countries can be added..
There is plenty of room for more countries. Remember when you look at the map you are not seeing the space available "behind the scenes".
Click <a href="http://www.themeparks.com/library-06/wdw/aerial/aerial-epcot.htm">here</a> for aerial view.