I hear you, and agree that FR isn't a lock. I'd say it is about 70% likely, with Greyhawk being 25%, and other being 5% (e.g. Exandria, Ravenloft, etc).
I was talking about a 50th anniversary Greyhawk product a few years ago, when the top of the 50th first started coming up. I would go big, like so:
Greyhawk 50th Anniversary Box
- 128-page world gazetteer
- 128-page ruins of Castle Greyhawk campaign
- 96-page city gazetteer
- 36-page historical booklet describing the original Greyhawk campaign
- Cloth Darlene retro map
- Cloth World map by Anne Meyer
- Cloth City of Greyhawk map
- Ruins of Castle Greyhawk cardboard cheatsheets
- Photo of Gary Gygax (just kidding)
Everything would be of high quality - all the maps cloth, sturdy faux leather covers for the booklets, etc. $99.95 price tag.