Hey all, I have been a long time DM and Player in Greyhawk campaigns. What I would like to see as a new GH product would be a campaign set in the distant past, for example; Rise of the Occluded Empire of Vecna, or during the Bakluni-Suel war. There is so much opportunity to create new products for GH if WotC just moves around in Eras. You want a campaign with High Magic, set it at the height of the Suel Empire. You want a campaign with the world at war, set it during the time of Vecna.
Since very little of this has been written, there is ample room for WotC to create something new that does not invalidate anything else that has been done.
In my opinion, the main difference between GH and all other settings is the WHY of its creation. GH was not created as a world setting, its main purpose was for EGG to run adventures and try out his new rules/classes/monsters/magic items/etc. He needed a blank slate to try all sorts of different things and see if his new rule system could handle it. Thus was born GH with all its different country compositions, its chaotic nature, and plenty of room to fill in the blanks. By its nature GH is designed to be able to handle any kind of campaign the DM wants to run.
There is a forum on Canonfire where some experienced [much kinder word than Old

] DM have put together a short half-page on many of the countries that describe the different kinds of campaign styles each one makes available to the DMs. You can find it here if you are interested.