RUMBLETiGER
I'd love to encorporate Warforged and Artificers from Eberron and most of the various races from any setting into the Greyhawk world. Living constructs could simply have a different origin then "House Cannith".
HA! XD (sorry, had to get that out...Fen will understand)
I've been talking with Fen via PM about adding Ebberon races and classes into the mix of things as well, and I think that, for the sake of uniform play, but against total isolationism, the other races, like Warforged, Shifters, Changlings, etc., should fall under the guidelines that Fen said about monster characters: that they'd be allowed to players with characters too high for lower-level interaction. That way, everyone can get a basic understanding of the game with the Greyhawk races, and then branch off an experiment with the newer races is there's a nitch they wern't able to fill, or an idea they wanted to elaborate on in greater detail.
As for classes, domains, spells, feats, skills, and other do-hickies and add-ons, that should be taken for a case-by-case basis.
Another way to throw things all together is to follow the main human principles regarding anything unknow: fear it. As such, warforced, shifters, etc. would be regarded in a slightly more hostile manner then the standard greyhawk races. This would also promote the use of disguises and what not as well, adding a little more flavor to the more mundane urban experiences.
Items from Ebberon and Faerun could also be considered, like Livewood and crysteel. To compensate, you could treat them like imports, and jack up the prices of items crated with them, or just to get them period.