I don't generally share Harbinger's dislike of the tofu race as it stands, but it does lead to humans being kind of bland in comparison to the ancient elves, the stalwart dwarves, and the myriad other races. I feel like, in Cerilia, the standard and variant humans are perfectly serviceable, but given the intense focus on the cultural differences of the humans of the setting, it is completely reasonable to present a handful of variants based on those cultures.
Each of the Cerilian human races has its own real-world cultural base, and it's tricky to do this in a manner that is sensitive to some of those cultures. While they are indeed fantasy cultures, the ones they are based on are certainly real, and you kind of don't want to be a jerk by stereotyping them in a statistical manner. Nevertheless, there are aspects unique to those cultures that can make for a compelling and enjoyable "Cerilian variant human" for player characters.
The humans of Anuire are "semifeudal and based on a class of free farmers and craftsmen", as defined by the Birthright rulebook. They have strong traditions and respect nobility and civilization, and the amount of politicking that goes on between the various classes of citizens means that most Anuireans know how to talk the talk.
As such, my proposal is this:
Ability Score Increase: Your Charisma score increases by 2, and one other ability score of your choice increases by 1.
Skills: You gain proficiency in either the History skill or the Persuasion skill, at your discretion.
Military Service: Most domains have a tradition of raising peasant levies to defend their borders, or training noble sons and daughters to protect themselves. You are proficient in simple weapons and light armor.
Revered Traditions: You are considered to have proficiency and advantage on any History checks made to identify heraldry, quote Anuirean laws, or recall historical events taking place in Anuire or as they relate to Anuirean historical figures.
Languages: You speak Anuirean and one other language of your choice.
This variant human loses the free feat, has a choice of two skills, some free weapon and armor proficiencies, and is an excellent scholar of history. They have a somewhat stronger ability score selection compared to other humans, and make excellent paladins and courtiers. The free ability score point also lets them add it to Bloodline if they so choose (of course, if you wanted to use the standard and PHB variant humans, they could add it to that too; this makes standard non-variant human slightly more powerful).
The Brechts are clearly based on the Mediterranean trading cultures of the early Renaissance, taking cues from Greek, Italian, and even some mild Turkish influence. They are the preeminent trading culture of Cerilia, and they are ruled less by nobles and more by the powerful trading guilds and merchant houses of the area.
Ability Score Increase: Your Wisdom and Charisma score each increase by 1, and one other ability score of your choice increases by 1.
Skills: You have proficiency in Persuasion.
Keen Eye for Value: You have advantage on rolls made to determine the value of an object, identify its origin, or haggle over the price of goods. This also grants you advantage on saving throws to see through illusions.
Languages: You speak Low Brecht and one other language of your choice.
Brechts have good ability flexibility and break the mold by having three different ability score increases (albeit minor). Keen Eye for Value has good roleplaying use, but also a powerful mechanic to combat illusions; it's hard to fool a savvy Brecht.
The Khinasi are somewhat unusual in Cerilian human cultures in that they do not generally fear magic -- it is indeed considered a noble calling, and Khinasi magic-users are welcomed in courts across Cerilia (save those of Vosgaard, of course). They have more "modern" cultural mores than the rest of the Cerilian nationalities, respecting gender and class of birth equally and with laws that treat all of their people fairly. In practice, of course, this may vary, but the Khinasi are an enlightened culture as a whole.
As a side note, if you can get your hands on the Cities of the Sun splatbooks (splatbox?) they are worth it for the cultural information alone. Beautiful stuff, kudos to Rich Baker.
Ability Score Increase: Your Intelligence score increases by 2, and one other ability score of your choice increases by 1.
Skills: You are proficient in one skill of your choice.
Magical Traditions: The Khinasi respect for magic encourages everyone to try to learn at least the basics. You may choose to know one from among the following cantrips: dancing lights, friends, mending, or prestidigitation. Intelligence is your spellcasting ability.
Languages: You speak Basarji and one other language of your choice.
Khinasi make awesome magicians, and even non-magical Khinasi know a trick or two (it's kind of like a pseudo-gnome, but Cerilia has no gnomes). Their Intelligence bonus may be a bit out of line, and I encourage discussion on that point. I can't say I'm sold on it just yet, but they are the most educated Cerilian culture.
Also, for a bonus in the next article, I'll take a crack at a variant for the other Khinasi subculture, the Masetians.
No bones about it, the Rjurik are very Scandinavian in theme. They are also the culture with the most druids, and their lands are rough and hardscrabble, steeped in ancient magic and populated by spirits both benign and hostile. Their worship of the Cerilian pantheon's nature deity, Erik, gives them a great respect for the forests and their inhabitants. I dare say they are the most likely to get along with Cerilia's elves, who are generally pretty hostile to humans (for good reason).
Ability Score Increase: Your Wisdom score increases by 2, and one other ability score of your choice increases by 1.
Skills: You are proficient in the Survival skill.
Respect of Nature: You are unimpeded by difficult terrain caused by undergrowth or the entangle spell. Further, you will always have a neutral reaction with fey creatures unless you or your companions demonstrate hostile intentions.
Languages: You speak Rjuven and one other language of your choice.
Rjurik humans make excellent druids and rangers, naturally. Their Respect of Nature ability can be powerful when fighting enemies in wooded territory, or when needing to negotiate with wild spirits. They might be a bit too specialized, so I'm not 100% on the Rjurik yet. I may "errata" them later on (as I may with any other presented material).
The Vos are as brutal and unforgiving as their homeland. The weak perish, the strong rule, and those in between suffer at the mercy of their tsarevos. Despite their barbaric traditions, the Vos have a strong code of honor, albeit one that calls for bloodshed at even the slightest offense.
Ability Score Increase: Your Strength and Constitution scores increase by 1, and one other ability score of your choice increases by 1.
Skills: You are proficient in Athletics. No Vos survives without rigorous physical training.
Savage: At the end of any turn in which you did not use the Attack or Dodge action, you may take a bonus action to make a single melee attack against any opponent in range.
Languages: You speed Vos and one other language of your choice.
Breaking the "rule" again by giving the Vos three minor ability score increases, and making them very heavily bent toward fighter-type classes. The Savage ability may be too much, and its wording may need to be clarified. Originally, I didn't have the Dodge action listed. You lose the ability to make extra attacks, if your class allows for it, but up until you get them, there's no reason NOT to Dodge every turn you're in melee combat.
It may be too edge-case without it, so I may take it out entirely on a future iteration.
So that's what I came up with for now. Next time, I'll hit half-elves, Masetians, do some errata on previous entries, and do some brooding on whether or not to scrap this entirely and go for racial feats as hinted in the Elemental Evil Players Companion (PDF link!).