Lots of people wonder why there are so many different types of humanoids in D&D; kobolds, goblins, orcs, hobgoblins, gnolls, bugbears, and (in AD&D) flinds, norkers, and xverts. They’re all clustered in the 2 HD-or-under end of the pool, and the overwhelming majority are lawful evil, so it’s not to provide a nice and comprehensive diversity among the population. There are just a lot of different species of humanoids out there.
In the original Greyhawk campaign, they were stuffed together cheek-to-jowl. Castle Greyhawk featured all of the various types in relatively close proximity. The DMG even gave us a table to tell us how one type of humanoid felt about another; hobgoblins tolerate bugbears, but bugbears actively dislike and will bully hobgoblins.
It occurred to me that a better explanation for the variety of humanoid races might be geographical diversity. Rather than the Caves of Chaos, with a tribe of bugbears a hundred yards from a tribe of kobolds, the humanoid races would be distributed geographically. In the southeast of the continent would be orc country, while in the north would be found the many kobold tribes. The further westward one went, one would encounter first goblins, then hobgoblins, and finally their fearsome cousins the bugbears. And so forth.
This doesn’t mean that there couldn’t be extraordinary individuals (or small groups) out of place, but they would be extraordinary. If you encountered a group of goblins in what should be gnoll country, it would be something to remark on and wonder why they were there.
I don’t have anything concrete with this notion, just throwing it out there as a possible way to give the otherwise difficult-to-differentiate stew of humanoids a bit more difference.
(Yes, I touched on this a few years ago, but some more thoughts sort of crystalized so I wanted to get it out there.)