There's actually plenty of open pavilion slots in the World Showcase to add more countires:
(Pulled this from a 2013 thread on another site, so other posters feel free to correct me if I'm wrong.)
It wouldn't be much of a lift for Disney Corp, since the space is already there, they just need to fill it. That's where the Imagineering staff comes in. Current representation: 5 European countries, 2 North American countries, 2 Asian countries, 1 Central American country, and 1 African country (Morocco) and one other African-type of "country" (I used quotation marks, since the country isn't really identified and it's pretty much a pass-through rather than an actual immersive pavilion).
Personally, I would love to see a Peruvian or Brazilian pavilion (they could have a steakhouse much like Texas de Brazil/Tucanos, where the watiers serve carved meats from a sword), Southeast Asian such as Thailand (mmm, Thai food), Australia/New Zealand (think of the sheeps wool merchandising potential!), Spain (the dancing!), and they could expand the UK pavilion to include more of Scotland/Ireland/Wales.