4E era Dragon and Dungeon publications weren't bad as we saw complete adaptions of Tamoachan, Tomb of Horrors, the Giants Series and several new modules not to mention a reprint of the Slave Lord series in its original AD&D format. As always I credit Chris Perkins a lot for this recent Greyhawk love. Wizard's organized play also got into the Greyhawk cookie jar and used among other storylines Beyond the Crystal Cave and the Ghost Tower of Inverness.
Before this run of course was Paizo's longer 3rd Edition tenure with the magazines where we saw expansions of Mordenkainen's Fantastic Adventure and a few epic APs and more original modules set in the Flanaess than I can name at the moment. Then of course there was the Expedition series modules. I wonder now, what's left of the World of Greyhawk's extensive library of modules (pre-dating 3e) that hasn't been reused, expanded upon or outright swiped.
EX1 Dungeonland and EX2 Land Beyond the Magic Mirror: These Lewis Carroll inspired modules aren't for everybody but comics, shows and movies with the Wonderland theme are in abundance so I can't see these as a bad option.
I1 Dwellers of the Forbidden City, I2 Tomb of the Lizard King, N1 Against the Cult of Reptile God, U2 Danger at Dunwater, U3 The Final Enemy: Perhaps some day there will be a renewed interest in jungle adventures and/or scaly folk as antagonists? Personally I've never ran any of these but I own them all. There has to be something worthy to use in all these adventures.
UK2 The Sentinel and UK3 The Gauntlet: These little regarded modules are set in a remote part of the Sea Princes and could make a good place to set expansions without much interference with canon.
WG6 Isle of the Ape: This may have been touched on in Paizo's Savage Tide AP but I know there hasn't been a major adaption or expansion done on this homage to King Kong. Again, jungle based modules aren't the in thing yet, however Greyhawk has gobs of them.
Crypt of Lyzandred the Mad, The Doomgrinder, the Star Cairns, WG9 Gargoyle, WG10 Child's Play, WG11 Puppets: Also known as the modules no one in their right mind would adapt. If anything they need gritty reboots with completely different plots.
Vecna Lives!, (Vecna Reborn) and Die Vecna Die: Not all technically Greyhawk modules but they all tie together. If I had to pick the next big Greyhawk thing to be exploited, Vecna would be it. He is an uber-NPC villain with a rich background, more than a few associated artifacts and multi-planar reach.
WG8: Fate of Istus: This module is in a class all its own and it's adaptation value is mostly in a city sourcebook. It could be interesting to do another Fate storyline however if we're talking something more Arabic in flavor this time.
WGA1 Falcon's Revenge, WGA2 Falconmaster, WGA3 Flames of the Falcon: This series based in Greyhawk City is undervalued and so isn't likely to ever be adapted. It still was a successful trilogy which is more than I can say for...
WGS1 Five Shall Be One, WGS2 Howl From the North: I loved the premise and set up of these modules but they were awfully written. It was so bad that evidently the third in the series was cut short and folded into the follow up Greyhawk Wars board game. People like Viking flavor, so this would be my personal choice for an adaptation or expansion module. The five swords could easily be scattered again and the quest renewed, but this time with a real goal in mind.