This is one of my favorite (and least favorite) parts of making RPGs!
On one hand, it's very fun making all kinds of different cultures and such, some heavily based on real world cultures, some combining various aspects from different cultures, some inspired by fictional cultures, and some just, I don't know where the idea came from, heh. And throwing all kinds of "non-humans" into the mix (elves, fairies, werewolves, minotaurs, harpies, robots, talking cats, dragons, etc.) is a load of fun!
But on the other hand, that can be very, very time consuming! Thinking all of this up, having it work in-game, not only designing the cultures for humans (well, not all of my ideas have humans) but for the other races as well....Like hey, maybe even though humans and centaurs live together and have the same culture and religion for the most part, they still typically have quite different world views...That also includes designing their appearances, which means lots and lots of drawing and tons and tons of spriting...
Oh...but it's so fun to create all this though...I for one like using some ethnicities in different ways too. Like, notice how elves are almost always European inspired? How about some Brazilian elves? Mongolian elves? Indian elves? Extra layers like that always just make things more complicated though! (I think it's worth the effort though)
Lately I've actually gotten a fair bit of inspiration from Magic the Gathering. It has some interesting settings with varied cultures, especially amongst the humans. And in some of the worlds, the ones called humans look a fair bit different from the humans you and I are used to. I always love the "world-building" articles they have around the time a new "block" comes out with a new world to learn about. Oh, would I love to see one of those "style guides" they have, all that detail about everything and all kinds of reference stuff, and hmmm...
Oh, and you know what else muddles the mixture too? Really "out-there" fantasy settings, like places with giant mushrooms and purple sky and stuff. How would the culture and society and religion of the people who lives there be?