Heh, it's funny. I'm currently doing a short campaign using The Infernal Machine. It's a super deep dive into D&D history - one of the parts is going to the Tomb of Horrors while it's being built and the other part is set in the Temple of Moloch (of the AD&D PHB cover fame). Great stuff and super deep dive into Greyhawk.
Now, according to canon, the Tomb of Horrors is set in the Vast Swamp. Fair enough. Not a whole lot written about this area. But, what interests me is Sunndi to the north. A fairly beefy sized nation that has always had a very high percentage of non-humans and is known as a refuge for non-humans. Other than that, there's virtually no information about Sunndi that I could find. Couple of paragraphs and that's about it.
For those looking for where dragonborn could come from, that's a pretty easy candidate.