I really like the idea of
@Snarf Zagyg message of not slavishly recreating and learning from the old setting to make it different from other settings. My two coppers would be:
- Lean in on the politics. I say this for two reasons. One, so each nation can be a little more xenophobic than modern settings. This, in turn, will highlight the species, class, and caste structures of each area. Two, it has the possibility for good RP from a player perspective. Align yourself with this kingdom and become renown; suddenly some parts of another kingdom possibly become off limits. Align yourself with that kingdom and earn renown; now the political leaders are asking you to do some shady stuff. (For the record, I think Mercer's Explorers Guide to Wildemount did a pretty good job with factions. It might be worth the writer's time to revisit that book.)
- DM's Guide to everything! There really needs to be expanded sections that are codified as rules the DM can implement for various settings - low magic being one of them. Nerf the heck out of the spells, it's an alternate setting. They really need to dismiss the balanced game approach for these alternate setting rules, and instead focus on how those settings can be made fun. This ties in to Greyhawk, as it will mean different things to different people. Heroic Fantasy Greyhawk? Sure. Just be clear how you're making those kingdoms heroic too. Again, don't worry about balance. Want to change the death rules for grimdark - codify it so younger players can experience something other than three death saves, a million opportunities to heal, and a hundred opportunities to be resurrected.
- Lastly, @TwoSix mentioned a Witcher vibe. I think this is spot on. To do that thought, I believe you need to reconstruct the daily encounter system. Maybe for Greyhawk, make it one to two encounters per day. Up the ante on monsters.
And as much as I like
@The Glen suggestion of isolationism and racial disparity, and how that can lead to my above suggestion, I think it will be a hurdle for most groups. To do that, you really need to species as a culture. Some species in 2024, like aasimar and tiefling are not a culture - they are unique individuals.