One point I'd make would be that since these are all Humans, but are just from different regions... I personally would make it such that no "physical" skills would be allowed to taken as proficiencies/expertise. So no Athletics, no Stealth, no Sleight of Hand, no Perception, no Acrobatics. Only "knowledge" or "communication" type skills would be allowed.
Reason being that all Humans should physically remain pretty close to each other-- especially when compared to the other races in the game. If the entirety of the Elves in the game world have Keen Senses (which is in comparison to the Human baseline)... then no specific sub-set of Human should also get it. And that same things holds true for any other physical trait or anything a Human is born with.
Whereas anything involving knowledge or communication is a cultural thing, and that makes sense that everyone in a certain region might be taught or just grew up constantly using. A Human sub-culture that lives high in the mountains might all know Survival because they had to in order to actually survive. A Human sub-culture in a theocracy (like for instance Thrane in the Eberron setting) might have everyone knowing Religion because their entire lives are spent getting hammered on its laws and practices. And a Human sub-culture that is warrior-based / the strongest shall lead, and you only gain power by taking it from others might for instance all be proficient in Intimidation, as that's the baseline action you hold towards others of your culture.
Those kinds of things I could see... but anything physical? To me it makes the disparity and differences between Humans to other Humans seem much greater than between Humans and Dwarves, or Humans and Elves. And I'm not so sure that's the kind of differences Human sub-cultures should necessarily have.