So
It's in gamefreaks best interest to pick regions that are popular in mainstream culture. They want you to be excited for the region just based on the region so they can sell more games and to do that they have to pick nations that are popular or cultures that are globally recognized. That's why all of the present regions outside of Japan are popular tourist destinations, and why I'll use tourism as the proxy for popularity:
https://en.m.wikipedia.org/wiki/World_Tourism_rankings1. Is France, which we've already gotten, but knowing that makes XY make more sense.
2. Is the US. And I'm sure Hawaii is in that figure too.
And now we get to countries that haven't had a pokemon game yet:
3. Spain
4. China
5. Italy
6. UK
7. Germany
8. Mexico
9. Thailand
10. Austria
So of these 8, I'd have to say that I'm the most interested in the UK, Thailand, Spain and maybe Mexico and Italy. The rest have some disqualifying features in my eyes.
No 1 is China. China has been trying to work off Trumps presidency by strengthening their trade relations even more, but the rest of the world is really very uneasy with China for their human rights record. And while Chinese history is fascinating, the nation itself has automatized itself into oblivion. There are riots about just how nasty the country has gotten, and it really doesn't fit with a game that celebrates the natural world.
Outside of that, any of the above would be good choices
