Many of the countries already mentioned would be good choices I think -- Russia, Brazil, India, Australia, Greece, etc...
Here is a piece from last year on adding to World Showcase.
https://dadfordisney.com/2015/04/21/will-we-ever-see-a-new-country-added-to-epcot/
It has a good map of the used and "available" slots (based on the original WS plan).
As explained in the article, several of the slots are already used for other guest space, now including the space between Mexico and Norway. The Outpost area (2 and 3 on the map) is probably the best place for a new pavilion. The author suggests that it would be preferable to locate an African country there (Ethiopia would be interesting I think), but with Morocco already in WS, the entire continent represented in AK, and representation of food and culture at AKL, I don't think that needs to be a priority consideration.
It's fun to think about the idea of filling certain slots geographically -- Switzerland between Germany and Italy, Spain between France and Morocco, Russia between China and Germany.
Thinking about WDW as a whole, South America seems the most under-represented area of the world, so I would prefer Brazil. Now, that could change for me if they were to say, oh I don't know, replace Dinoland at AK with a new land based on South America. If that happens, or if they could just add more countries, my next choice would be India followed by Australia or New Zealand then Russia.
Less relevant Epcot ramblings follow (read at your own risk)...
Thinking about priorities at Epcot, it would be nice to see them overhaul the Future World area before expanding WS. Start with a re-imagining of the Wonders of Life pavilion, including an attraction based on Inside Out. Then figure out something to do with the Innoventions area and the theater at Imagination, and update the Energy attraction. Actually, it would be fun to have the same cast with an updated show in which Ellen walks in thinking she has all the answers based on the current outdated show, only to find out that she has to change her thinking based on updated knowledge and technologies.
ETA -- Here is a world map showing which countries are currently in the WS.