Norway and China are the only two countries that "touch" - all the others have empty plots between them (if you count World Showplace and International Gateway as things that can be reworked to allow for new countries - which they are).
The "do next to nothing and it's ready for a country" spots:
Mexico / Norway
China / Germany 1
China / Germany 2 (that area is big enough for 2 pavilions)
Germany / Italy
Japan / Morocco
Morocco / France
Also, there is expansion space on either side of the American Adventure - it can accommodate a pavilion roughly the size of like Italy or the intended pass-through promenade to a second lagoon or expansion area as needed. In theory this was where the original Epcot resorts where to have attached - along with the Australia and South Seas portions of the World Showcase, not at International Gateway.