I think at MOST we'll get one new country in EPCOT anytime soon. They've done a large building job at Magic Kingdom, Downtown Disney & Animal Kingdom are currently ongoing. Hollywood Studio's is in need of work and is probably next in lie, but after all that they will get around to EPCOT.
Anew country is a far bigger marketing opportunity than a new attraction(s) in the existing countries or improving bits of Future World.
So let's look at the possible countries first 6 that won't happen
1) Russia - Currently very anti-America and vice versa, the chance to build Russia was during the late 90s but has passed
2) Israel - They are not going to build a pavilion dedicated to a country with a nearly permanent war.
3) Africa - If they suddenly move from countries to a full continent it seems a massive change plus which there's already a Africa section in Animal Kingdom
4) India - Once again already exists in Animal Kingdom, there is no benefit in duplicating an area.
5) Saudi Arabia/Arab Emirates - Morocco seems to have become a catchall for this entire part of the world already so you would be taking away from the Moroccan pavilion to build this one.
6) Spain or Portugal - Different food options not already covered, however architecture and culture not massively different from other existing countries
Now 5 that could happen.
1) Greece - Unique food that isn't anywhere else in Disney World, lots of history and unique architecture to work with and an existing Disney movie to work in.
2) Egypt - Not really touched on in the Africa areas and is unique enough to stand out. Lots of unique standout architecture and mythology to work with.
3) Brazil - Huge marketing opportunity south of the border and unique food/culture to work with along with existing characters. However the characters are already in the Mexican pavilion.
4) Australia - Huge untapped theme with a large untapped market, lots of opportunities. This seems the most likely fit unless it is left out to be placed in Animal Kingdom at a later stage.
5) Switzerland - Already planned out with breath-taking scenery that can be used, everybody wants the Matterhorn ride.