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Spain is probably the best just because it has effectively seven different nations already within it. The differences going from locales like Galicia all the way to Andalus are pretty dramatic. Barcelona is also just too much of a "global draw" bait to ignore. We have to remember here that this is Japan who is doing this sort of design. This is the nation of people who operate on stereotypes, created a veritable "Galapagos isles" effect for technology across their entire archipelago by coding everything electronic (even fucking digital watches) in assembly only stopping once apple products arrived, and wore jean jackets into the mid 2000s. They need all the help that they can fucking get when it comes to making a fictional analog for a foreign nation look like anything more than a single stereotype extrapolated over a titanic landmass.
Spain has the variety only because it's not just obvious, but the Japanese probably know enough about it to recognize the variance in that nation's terrain and populace. If they adapted a region for Mexico, it would basically just be a bunch of blanket-wearing, sombrero-shaded, tequila-sipping vagrants in dusty towns. Moreover, they probably don't know shit about the rest of Meso/South America to develop anything remotely accurate or interesting (aside from MAYBE Brasil). Regardless, they'll probably make it about as shitty as Galar, so it doesn't matter which locale that they adapt: it's probably going to be just as fucked as gen 8.