Greyhawk Wars, From the Ashes, et. al. - I didn't really get much of interest out of those, to me they changed the tone of the setting (due to different authors etc), then what was done in 3.x was OK but again didn't really do much except give a context for the core books. Which is my point really - "in the old days", Greyhawk WAS AD&D and visa versa. Sure, you could create your own world, but read the AD&D Dungeon Masters Guide, Players Handbook, etc, and they are littered with chunks of Greyhawk. I've got the original Greyhawk Boxed Set, as well as the Gazetteer, and while the Gazetteer is good the original boxed set is more than enough to give me a set of ideas to help fuel campaigns for a lifetime.
For many, the whole appeal of Greyhawk was that the amount of material written about it, especially the amount of specific information, was very scarce. So as a DM, you were forced to fill in the very considerable blanks, using what you might find for inspiration.
Contrasted to the Forgotten Realms, where nowdays if you dig around a bit you can find a very detailed map of nearly any part of the Sword Coast, detailing all sorts of dungeons and villages, trails and roads, adventure modules, you can find novels, game supplements, etc etc etc. Yes, that level of detail can be useful at times, but it's also very dis-empowering too.
So for me, I struggle to see how any modern re-interpretation of Greyhawk could do anything useful... If it's rules focused, it will likely miss the point because the "Greyhawk Rules" were, in fact, the rules of AD&D; trying to put old AD&D rules into 5e would be completely silly, and beyond that I struggle to think of what's missing from the core 5e rules that you'd need to have fun in Greywawk - the only thing most people are missing, is players who have a working knowledge of the campaign setting. If it's "fluff" focused, I think that Forgotten Realms has gotten so big now, that whatever you could come up with for Greyhawk would seem lame to a modern audience - it could easily seem (ironically) derivative, and/or fail to differentiate itself from what is now a very popular and well known setting - despite how us "old timers" love Greyhawk, there's nothing much that makes it a compelling proposition for your campaign, excelt the very simple fact that it is The Original (i.e. despite what Gary, Dave Arneson etc played in their home games, the World Of Greyhawk is what got published, well before any other settings came along; it has elements from all sorts of people's own campaigns and adventure modules in there).
