I'm shocked they haven't added anything in so long.
I like the India idea. It's the second most populous country in the world, has awesome food and could inspire a WS Taj Mahal (although I've thought about this before and am afraid it would be a disappointment because it would have to be shrunk down so much). The downside is that there is already a strong Asian presence in the Showcase.
I love the Greece idea. Again, the food is awesome and, as others have said, theming could be fantastic. Also like others have said, though, the European influence is also super strong in WS.
Russia seems like it would be fantastic. Now that we're well past the Cold War, it could be a big winner. Still, I'm doubtful Disney would ever go there.
I think that means it should probably be a South American country or Australia. Brazil is the obvious choice due to its size. I admit I don't know much about Brazil; my guess is the majority of North Americans don't either. The only super famous Brazilian landmark I know of is Christ the Redeemer, and something tells me that is not going to be transplated to Epcot. Still, I bet the cuisine and culture could be presented in a fascinating and fun (can you say Carnaval?) way. The other huge plus going for Brazil? They're hosting the Olympics in 2016, so the entire world will see and learn a lot about Brazil over the next several years. That should pique the interest of many and could be a good lead-in to the newest country in WS.