In my own homebrew world I have a myriad of cultures (based off various known cultures).
But they were not created in a bubble.
You can have "gothic-noire" style games/plots/regions and "high magic South America" with a "high fantasy Arabian-flavored nation" and "european medieval realism", "eastern" philosophy or feudal make-up with "pseudo-rennaissance" levels of technologies all inhabiting the same world....heck, laser blasters and "space/spell ships" if you want.
Sure you can.
But these "ancient tribal peoples" using bone-tipped spears a week (or even a month's) journey from where an "Ottoman"-style empire have guns and flying ships that abuts the "Count's evil-infested realm" on the other side of that hill...just ruffles my feathers.
Develop worlds that "make sense." Where peoples and nations have interaction (or not) and influence each other (or have some reason not to: xenophobia, cultural taboos, varying resources or the competition thereof, generational "hard feelings" from some unforgotten -or ongoing- war/atrocity, etc..). Common ancestries/origins, intertwining histories, complementary (or competing, but not utterly unrelated) religions or pantheons, utterly foreign language branches and disparate levels of technology. How many "hidden valleys" and "lost/forgotten kingdoms" can one world contain?
Yes, obviously, it's a fantasy game of imagination. The answer, technically, is "as many as I want." I understand that.
Yes, you can
and should have diversity. Who wants to adventure in a world where everything is the same? blech. What's to explore in a world like
that?
But I don't want a setting that looks like or, perhaps more importantly, makes me
feel like I'm in Epcot Center.
We just defeated this demi-plane/pocket dimension seeping into our world of vampires and werewolves. Tomorrow? After a night at the inn, you're in the desert sultanate being controlled by a djinn and invaded by sandstorm demons. Two days later, you're swashbuckling on the coast with sidearms...but the desert people never saw a "boom stick" before and the clerics of the sea-faring populace's Ocean God can detect/effect a vampire a mile away...with their solar-powered photon disruptor maces that channel the sun...?
Variety is the spice of life to be sure. And all of these have a place and are/can be great fun. But, trends of the music industry aside, "mash-ups" aren't necessarily the best way to build a "believable" world/setting...imho.
I think the suggestion of "homebrew" as the default with guides and tips for all of the various elements of setting creation is the best idea/way to go.
"DIY/Make your own as you go" should be[come] the norm for those who don't prefer to use an already published world. And really shouldn't be as "daunting" as many seem to feel it is.
Have fun and happy "default setting"-building.
Carry on.

--SD