I would eat up a hardcover, deluxe edition of Greyhawk like a fat kid eating cupcakes! The Living Greyhawk Gazeteer is an interesting piece of work but reading it is worse than reading a high school geo book and it has little in way of 3e snippets. I think WotC would be wise to publish a 3e Greyhawk book with the Greyhawk specific prestige classes from the class books like the Knight of the Great Kingdom, Eye of Gruumsh etc. and take the chance to revise the setting description to be a bit more interesting, give a nice detailed account of the core deities and the Great Wheel cosmology, NPCs etc. If done right I think WotC could have a knock out product for Greyhawk that makes fans turn their heads and SEE that Greyhawk isn't bland and boring and is quite exciting. Look at the Pomarj! The intrigue in the Great Kingdom! The classic Dungeons! Instead of publishing sourcebooks like we get for FR Wizards could publish adventures that flesh out towns and such instead like in the 1e days. Think about the imagery of an adventure Called Death Knights of the Flanaess! Urpising in the Pomarj! The Scarlet Brotherhood strikes! Some of the most popular modules in 3e from Dungeon have taken place in Greyhawk! And WotC shouldn't worry about stepping on the toes of the regions in Living Greyhawk. Its just two different campaigns!
I would actually like to see WotC do more settings, re-releases and original material. Without an overreaching metaplot to their products D&D has only so many sourcebooks that can be done. I think they should continue ongoing support of Eberron and FR and then release a new setting once a year with a core book, a races book and a magic book and some adventures. In this way they keep things interesting and fresh and at the same time they aren't over committed to products like TSR was with their four concurrent campaign settings plus one or two settings that inevitably got cancelled within a couple o years (Dark Sun, Birthright, Mystara etc).
I would like to see a World Builder's Guidebook to help those DM's who want to create their own settings from the ground up and approaches th concept from the two different directions, helping the DM flesh it out as he goes along. It would include an adventure primer etc.
More genre series of books.
Planar adventures, even if not Planescape, Planar adventures are very popular adn something that many a DM loves to run because of the sheer amount of imagination they can put into these adventures.
Jason