I'd certainly start with the HUMAN cultures: Flan, Oeridian, Suel, Baklunish, Rhenn, OA-Asian, Erypt, Amedioan, Hepmon, etc.
[snip]
View attachment 254829
Oh, hey! I colored and labeled this. It's fun to see it pop up in the wild!

(The original b&w illustration is by
Vince Locke, from the
Living Greyhawk Gazetteer for 3rd Edition, 2000)
I think in regards to a
Greyhawk setting book, I'd like to see the principles of Arcanist Press's
Ancestry & Cultures applied, splitting up inherited traits and cultural tendencies in order to get away from old bio-essentialist concepts and make things more interesting.
Sensitivity readers would be a must; given the number of cultures in Oerth that are based on real-world ones, knowing harmful tropes to avoid in the text is a necessity going forward. So many of the things in
Greyhawk were inspired by pulp-era novels (the famed Appendix N from the 1st Edition
AD&D Dungeon Master's Guide) and adventure films
(WG6 Isle of the Ape), and while there is a discussion that can be had about Gygax's personal views on such things, to what degree he was a product of a bygone era, etc., that's a topic I'm not educated enough about the man himself to drill down into. As far as the gameworld he created is concerned, I think we can strip out any offensive stuff (unintentional or otherwise) without losing anything that makes the setting fun.
It is possible that the "Suel" in the continent of Flanaess, roughly corresponds to reallife Nordic ethnicities. They are often depicted as having light complexion and the silliness of "viking horns".
There are difficulties with using Suel to represent Nordic ethnicities − besides how Greyhawk demonizes these ethnicities. Suel is decadent, cruel, and conniving. The N*zi Scarlet Brotherhood are said to preserve best the Suel culture. One can see why reallife Nordic peoples object to such an identification! The problematics continue. The Suel breed mul (≈ derro) as a dwarf-human slave hybrid. Likewise they engage defacto racist wars against orc, goblin, and hobgoblin, tho to be fair, Greyhawk characterizes these as more like nonhuman fiends, they remain ambiguously resembling reallife ethnicities anyway. The setting supposes that Suel is a specific prehistoric empire on the cusp of history in a location that roughly corresponds to the great deserts of Asia, such as Gobi Desert and Taklamakan. (These Asian deserts collide and blend with Mexican deserts.) In a later war, the Siberian-esque Baklunish unleashed the genocidal weapon of the "Rain of Colorless Fire" that causes the desolation of the Sea of Dust and genocides the Suel Imperium. During the various wars, many Suloise had already fled into the areas corresponding to North America, forming a Suel diaspora in North American Flanaess. Yeah.
[snip]
What makes this Greyhawk setting so violently painful to Norwegians is: the N*zis invaded us. The WW2 Germans did evil against Norway. They are not us.
The Nordic peoples are not − and have never been − Germans!
The concept of a socalled Aryan race is a bygone German scholarly racist fiction. There is no such thing!
That antihuman crap has nothing to do with us!
This is a fair amount of what I mean. This is a case study in why I think sensitivity readers should be a priority for any future publication ambitions. I'm reminded of Professor Tolkien's annoyance at the N*zis' erroneous and racist use of the words "Aryan" and "Nordic" and how he responded to a German publisher during that time asking about his ancestry before publishing
The Hobbit (short version: he politely let them know they were ignorant racists and wanted nothing to do with them).
I've interpreted the Suel Imperium as something akin to Moorcock's Melniboné — probably between the decadence and cruelty of the late Imperium and the physical resemblance of the Suel to Elric himself — that ended up effectively destroying itself over its long-running hatred/rivalry of the Baklunish Empire. As a child growing up in the 80s, the Mutually-Assured Destruction aspect of the Twin Cataclysms always felt like a grim warning about the Cold War to me (present in many other RPG publications at the time to one degree or another), and certainly not an act being celebrated in the text. It also struck me as a necessary "Civilization Collapses" historical note that you would need in order to mirror the historical collapse of Rome leading to the Medieval era
Greyhawk tries to emulate. With the old empires in ruin and their peoples scattered to the winds, you have a world that's filled with lost artifacts and haunted ruins of a bygone age of power lost to myth.
But even the Imperium wasn't necessarily
always decadent and evil, and perhaps it would be good to mention that fact in the text, if for nothing other than to serve as a pushback against bio-essentialist notions.
As far as the Scarlet Brotherhood goes, I appreciate having them as a villain organization in the gameworld for the same reason there are N*zis in the Indiana Jones movies. The Suel as a distinct people spread out across the Flanaess during the Great Migrations and settled down all over the place, intermarrying into other ethnic groups and creating new cultures from the blending. Gygax's choice of wording about "purity" in the old publications is not something I'm going to defend, but the Scarlet Brotherhood paint themselves as the preservers of the old Imperium, and I don't see any reason to give them the benefit of the doubt about their claims. Given the circumstances of the Twin Cataclysms and Great Migrations, I think it's reasonable to conclude that their understanding of the Imperial culture they're trying to preserve is as much of a fiction as the N*zis' beliefs were. In my campaign I ultimately plan to have my SB antagonists confronted by a lich from the old Imperium who denounces them as delusional and corrupt before trying to destroy them, demonstrating to everyone through as authoritative a voice on the subject as still exists that these guys are fooling themselves.