Henadic Theologian
Legend
WotC has cleverly positioned Greyhawk as holding different genre assumptions from the Forgotten Realms, so combining that nostalgia factor with "old timey Swords 7 Sorcery [cleaned up for the modern day]" as a genre booster, on the 50th anniversary...it just fits as a reasonable possiblity.
Mike Mearls kept pitching that idea, but it never really fit the setting IMHO. WotC has some major Greyhawk fans, but they keep pushing what I've seen other folks call the "Greyhawk Refugees" into the Forgotten Realms and now the Domains of Delight. They've pushed so much Greyhark into FR if they put anymore in, they might as well make Oerth a second moon around Toril.
They wouldn't do that if they had any confidence they were going to visit Greyhawk. It's pretty much only been Greyhawk this has happen too (and Nerath and that started back in 4e, perhaps because Nentir Vale was originally intended to be part of the Forgotten Realms, so I view it as something of an offshoot of FR, more then subcampaign, but something FR can poach from freely ).
Acerarak, Vecna, Mordenkainen, Tasha, and possibly others. Actually given what they did to Great Wyrm Dragons, I suspect some of Greyhawks most powerful Dragons will be found in the Forgotten Realms as well now.