I've been thinking of designing a region like this myself but I'd love a region based on Florida weird as it sounds. We have a lot of interesting wildlife here which can be used as Pokemon inspiration (like manatees, skinks, horseshoe crabs, armadillos), and swamps and wetlands as environments have potential. There's also a lot of opportunity for a story about how tourism and large scale development can have negative impacts on the environment. While they already did this for Hoenn (granted not that clear that they were doing that), Florida is pretty much like a real life human development vs. nature conservation.
That aside I'd rather they don't do another European country, maybe somewhere in Africa or South America, and I think an African country would be really cool given the amount of diverse options and how African architecture is usually distinct and look really cool, and my first thoughts were Ghana and Egypt. Granted they'd probably go for a bigger country before that and I've seen a lot of really cool fan designs with India.