ditch humans
ditch multiple races
one very humanoid race, human-like in intellect and tendencies, few big physiological differences
base cultures around those differences first
>they look vaguely like humans, but have horns
>in Examplia horn size and intricacy is a basis for caste system
>Samplians, however, historically file their horns down to tiny stubs
>this has naturally led to a lot of mutual mockery and often even wars over the course of history
next take some important philosophical or social aspect and make it like really important in the setting
say, let's take guile
>Examplians believe that true guile is represented in everyday affairs, in cheating everyone surrounding you in smaller things. This leads to a competitive spirit and keeps damage to a minimum.
>Samplians for instance believe that only serious deceptions are worth attempting, and in executing - or seeing through - them is true worth found
last, make something in the world that ain't a big deal in our a big-big taboo. a nation is considered savage when it doesn't uphold that taboo.
for us it could be narcotics. or female nudity.
for them it could be, say, sweets. or stimulants of any kind, including caffeine.
if you're feeling ballsy you can omit some taboos from other world. ye, their females walk topless, it isn't even seen as a sexual thing, jeez (okay, kidding, that's inviting erp and magic realm, but you get my drift)
obviously various nations deal with taboo differently.
with these alone you can make solid nations, and fill in other details from our world. players will probably recognize the expies, but as long as you keep focus on the made-up differences, it won't matter as much.
oh, and you can use some political things that either never worked or are completely outdated in our world.
Examplians, for instance, have strict case-based democracy, kinda like ancient Athens.
Samplians, on the other hand, are anarchists! They rely smaller groups sharing responsibility.