I'm really curious what kind of rules set it could be. Unlike Ebberon and Dragonlance, Greyhawk doesn't have anything that's "missing" from the existing rules set. If anything, DM's need to figure out what needs to be removed instead. I suppose "old school" type optional rules could be done, but most of that would be applicable to all AD&D style gaming, not just Greyhawk.
If I had a wishlist, it would be:
* Druidic (Flan) Bards
* Tallfellow Halflings
* Greyhawk Gnomes (as opposed to the Tinker Gnome in the PHB called Rock Gnomes and Forest Gnomes which aren't suppose to exist in Greyhawk)
* XP for Gold and Magic Items (reducing monster XP)
* Grey/Faerie Elves (rather than just High Elves)
* Valley Elves (rather than just High Elves)
Grugarch/Wild Elves (rather than just Wood Elves)