Region 1 is the Sheldomar Valley. The natural boundaries that surround it render it relatively isolated and thus perfect for a self-contained campaign. On three sides there are mountains, and water on the fourth. It features a single dominant political entity (the Kingdom of Keoland), which is itself surrounded by independent and, in many cases, resentful states. On its very fringes, the mountains hold many secrets (the Against the Giants series was set in the Crystalmist/Jotens/Hellfurnaces range, of course, and there’s the spaceship from Expedition to the Barrier Peaks, not to mention the Valley of the Mage itself as a destination for adventure).
Region 2 is the central Flanaess. This, of course, is where Gary Gygax rooted his own campaign, and it shows. It has a great variety of terrains, from woods to plains to desert to marsh, plus the Rift Canyon. It is thematically centered around the city of Greyhawk itself, with Iuz (circa 576, remember, with Iuz only recently freed from the ruins of Castle Greyhawk and still “Old Iuz of fearbabe-talk”) as a distant but still relevant enemy to be invoked. The whole revolves around the Nyr Dyv, and the tensions originally described in the World of Greyhawk folio set are really a great environment for an adventure. There’s the city of Greyhawk, Castle Greyhawk, White Plume Mountain, and the Village of Hommlet/Temple of Elemental Evil. Again, not an accident.
Region 3 is South Province and the Iron League. This has intrigued me ever since the “State of the Flanaess” articles penned by Gygax and Kuntz back in the early 1980’s in Dragon Magazine, which portrayed the slow and deliberate march to war between the two powers (with the added bonus of the rivalry between South Province and Medegia, itself complicated by the looming presence of the Overking in Rauxes). This would make a great place for a military-themed campaign, or even a stand-alone miniatures campaign.
Region 4 is the Aerdi Sea, and again it’s doubtless no coincidence that this region was originally Len Lakofka’s Lendore Isle campaign. The tension between the Spindrift Islanders and the Lords of the Isles, against once again the forces of the Great Kingdom (itself split between Medegia and the Sea Barons) would make a terrific place for a long-term nautical themed campaign.
And, finally, if you want Vikings, look no further than region 5, the Thillronian Penninsula. Put together five competing barbarian nations, and several struggling outposts of civilization under their onslaught (said civilized kingdoms very well defined, such as the Pale and Tenh), and you’ve got the makings of a terrific campaign.
This isn’t to say, of course, that one couldn’t start up a campaign anywhere in the Flanaess; one of its strengths that has stood the test of time is its versatility. But it’s also a testament to both its origins as a patchwork of several campaigns of its own as well as the guiding hand of its overall original designer that it works so well on several levels.