I bought the Greyhawk folio in 1981 because I thought the poster maps were cool. Plus it was the official D&D world and I wanted to get in on that. I loved the country descriptions and coats of arms and thought it was a really good product. The later boxed sets were cool too, but they were a lot more and bigger and may have crossed a line somewhere into too much. For me the folio was perfect and all of my 1st and 2nd edition games (and much of 3rd ed as well) were set in Greyhawk.
On to 4th edition: So the new races and the multitude of classes has no sensible way of being shoved into CY500's Greyhawk. There's just no way - we don't need any planetary collisions here. Greyhawk is versatile - heck I ran Fantasy Hero in Greyhawk for a while, and GURPS too. But 4th makes a lot of changes to what is "standard" and they don't mesh well with what was standard for 1st edition - and I want to play a standard, wide-open 4th edition game.
I thought about a timeline jump forward but that still leaves one asking where did these dragonborn things come from? Where did the gods go? Where are my elemental planes? So I went a different direction - literally. I decided to jump a few thousand years backwards.
Now if we go back about 2000 years we can use the Greyhawk maps and some of the "touchstones" - characters and names without having to retcon in a bunch of weirdness with races and gods. These are the early days of Greyhawk, the Time of Legends - Bahamut is a major god and the planes are different and tieflings and dragonborn are everywhere, but it won't always be this way. My idea is that this is the age where many of Oerth's lesser deities and Demigods ascend to power (since that's one of the Epic destinies in 4th). Some of the gods that exist in 4th will be destroyed, some will be changed in the intervening millennia. If some of my players end up as demigods, I will use that when my apprentices get into 1st edition AD&D and start exploring Greyhawk.
Bahamut will sacrifice much of his power to bring order to the universe - separating the elemental planes, and creating the ethereal plane as a barrier around the prime, and summoning the Deva race into the Astral plane to be his servants. Dragonborn will ascend to the planes to serve Bahamut and those that do not will hide in far places or die out. This gives the lawful good divine types a chance to be part of a Divine Plan - their god has goals and is carrying them out. This will make for a nice contrast with most campaigns where the evil gods are the schemers and the good ones have to react.
The Raven Queen may evolve into Wee Jas or she may get sacrificed and become the Negative Elemental Plane while another god may become the source for the Positive plane, or maybe they are already there but hidden.
I may retcon Goliaths as "Half Ogres" - it fits GH better but I haven't read the race entry in the PHB2 yet.
Eladrin will eventually lose some of their powers and become simple elves, but in the time of 4th ed they are still at full strength.
Tieflings will eventually be thinned without the concentration of an empire and will disappear as a separate racial type.
The rest of the races already have homes in GH so need no adjustment or long term consideration. Dwarves will lose their connection to most Arcane magics. Magic itself will become weaker and more difficult to access, meaning it takes a magic-user to tap into what were once rituals anyone could perform.
The idea is to use whatever published adventures are out and I like, with the freedom to place them anywhere in the world. The world should have some familiarity but it is not the "old" Greyhawk. It should accommodate all the 4th edition crazyness and give us some "legends" to incorporate into later versions of GH. I gave some thought to doing my own world but this is pretty much a blank slate on an existing map - and even that I change up. Maybe the Isles of Woe are yet to be sunk? Maybe some of those artifacts in the back of the old DMG are new or in the process of being made?
A note on Vecna: Yeah, he's a god in 4th ed but he's just a legendary figure in 1st ed and trying to become a god in 2nd ed. How to make that work? Well, just because he's a god now doesn't mean he can't fall - this is another plotline to follow with a campaign. The ups and downs of various gods and their followers should be a lot of fun at Epic levels of play.
So in the end, this is my choice for a wide-open campaign for 4th edition. It would be a fair amount of work, but there's not a lot of stuff to rally change from the basic 4th ed game.