I have discovered, with the arrival of 5E, a strange urge to reclaim the pieces of this patchwork, and try to weave them together into something resembling a persistent and connected setting, which I can use going forward as background for diverse games without having to throw away everything and start from scratch over and over. This is part of a larger rekindled love for the game, which I have to admit had been struggling, and I am very happy to see returned. I imagine I'll explore the why and how of that aspect later, but for now, I'm looking at the campaign worlds and how they can work as a unified world.
The Ones Still Left Behind
Not all of my games can be united this way. Eberron is too distinctively itself to be put into another setting, and the fate of that particular campaign is still TBD. Fortress of Tears, likewise, cannot be simply transplanted. The cosmology and structure of that game was designed to be a cohesive whole with a very specific feel, and while I doubt I'll do anything with it any time soon, I think it deserves its own space on the shelf, so to speak. The tongue-in-cheek setting of "Mid-Evil" is not itself going to be integrated into the world, although bits of it that I liked might be imported. Finally, my Fantasy Hero setting was too far removed from the premises of a "D&D-land" style setting to really integrate, so it also remains its own thing.
This is the setting as it currently stands. Racial/cultural notes are broad strokes and not intended as a straightjacket: it's a wide, colorful world and there are enclaves of different cultural groups in every major city. Demihumans and monstrous humanoids have been largely left out of the discussion because for the most part I don't have a whole lot of different cultures for them worked out and I don't want to put in any limits I don't have to. Assume the usual baseline for Dungeons & Dragons for such folk unless you have a reason to do otherwise. Campaign-specific races (such as the nephilim from Zan-Xadar or sirfox's gnoll cleric in Red Hand of Doom) exist, but are local populations rather than world-defining ones.
The Silver Coast
Bringing the world together begins with The Silver Coast. Argent is on the northwest edge of a large continent which is bordered by several seas and island continents to the south. Not too far north of Argent the climate quickly becomes cold, and there is a large glacial "land mass" that expands and contracts with the seasons, connecting the main continent with an arctic continent for half the year (being a cold and stormy sea for the other half). Legends tell of a zone of permanent warm paradise in the center of this arctic zone, but you know what legends are.
South of Argent is the kingdom of Ertikan. North and northeast of Argent are various "barbarous" (by Argentile standards) peoples, the friendliest of which are the blonde- and red-haired Calladgangers, but most people of the northwestern part of the continent tend towards fair or ruddy skin and hair colors and curly or wavy locks. The religion of this area resembles the core religion of Faerun, unless specified otherwise, although the gods of Oerth are also known here.
Fellhollow/Rise of the Argent Lord
That game (all two sessions of it) took place 600 years ago; Argenti, "The Silver City," was later rebuilt as Avileigne... only to have Mt. Thunderdelve erupt and destroy it. That place can't catch a break. (The modern city of Argent takes its name from the older one, even though it is actually some thirty miles to the northwest. Sort of a "New Argent," as it were.) The town of Fellhollow was destroyed during the orcish invasion known as the Rise of the Argent Lord; the later town of Pelann was built near its ruins (and, like Avileigne, destroyed again by the eruption of Thunderdelve).
Red Hand of Doom/Revenge of the Giants
A wide, grassy plain on the eastern side of the Silver Ridge Mountains eventually connects to "The Endless Plain" in the northwest corner of the Elsir Vale map. Most of the plain is sparsely populated, but there are nomadic horse tribes related to the Calladganger peoples (think Rohirrim) and at least one large civilized city-state (as yet undefined) lies by a large freshwater lake or inland sea on the one major road between Argent and Elsir Vale. The events in the Silver Coast game are concurrent with the indefinitely-hiatused Revenge of the Giants game (thus, Elsir Vale is currently in the midst of a deep and unnatural winter), and it is conceivable that the two games could connect in the future, allowing characters to overlap. The people of Elsir Vale tend to be of fair complexion, with thick, dark hair, although much variation is common. The religion of this region resembles the core religion of Oerth.
The Greyhawk Campaign/Shadows of Thessalaine
The country of Thessalaine, which bears a remarkable resemblance to a similar country on the world of Oerth named 'Bissel', lies to the northeast of Elsir Vale, beyond the Giantshield Mountains (referred to in Thessalaine as The Barrier Peaks). Some years ago now, the infamous necromancer Evard the Black attempted to conquer Thessalaine, but was eventually defeated by "The Watchful Seven" (a group whose variable roster most reliably included Kyriela of Kithria, Jaer, Dragor, Angelina, and Verdhaven). Thessalaine has a very diverse population, being a sort of "crossroads of the world". The religion of this region generally also resembles the core religion of Oerth. To the east of Thessalaine is terra incognita for the moment.
Zan-Xadar, Jewel of the South, City of the Wicked
Zan-Xadar is on the southernmost tip of the continent, far southeast of the kingdom of Ertikan and south of Thessalaine across the Desert of Xadar, as is its neighbor/rival city-state of Khaldun. The island city-states of Kithria and Nellevar are in the warm Opal Sea, south of the continent, and the near-mythical (to the Silver Coast, anyway) nations of Alcairam and Setranophis are on the northern lip of a vast continent far to the south populated by nations largely unknown to the north. The one adventure we actually ran in Zan-Xadar ("The Fallen Fortress") can be assumed to have "just happened" if and when a future game takes us back there. The peoples of this region tend towards darker skin and hair colors, with dark brown or black skin dominating Alcairam particularly. The people of Setranophis are a distinct racial group with reddish-brown skin and very fine black hair. The religion of this area is a crazy hodge-podge of cults from around the world, although the worship of Bahamut, Tiamat, Methis/Erathis/Titania, Nergal/Garagos, Baaltis/Ioun, Fortuna, and Kelaeno (a.k.a. Mother Hydra, a.k.a. Umberlee) are prominent in the great glittering cities. Setranophis and Khaldun have sanctioned state religions, an ancestor cult and the Goddess of the Black Flame respectively, and the worship of other deities is strictly illegal in both places. The people of Zan-Xadar and its neighbors are cosmopolitan and sophisticated, and regard the people of the northern realms (with the possible exception of Thesselaine) largely as bumpkins. (Thessalaine replaces Beltharain from the original Zan-Xadar setting; heck, they even sound similar.)
Castle Strongstone, the "Tower of Power", and the Tomb of the Zodiac
These are places of myth and legend, associated with the tales of the great human wizard Mystic the Strange, the wily elf-rogue Fgyarbt, and the gnomish trickster Zarfbardafardwards. It is generally believed that their adventures took place in a lost country somewhere north of Thessalaine or Elsir Vale, although many lands claim to be "where it really happened." The objective truth of the matter has long been lost to the ravages of eons; who or what a "Slick Rick" might be, no man may say.
The Empty Spaces Between...
Assume that unless it is explicitly states otherwise, there are other nations in between the ones listed here which have simply "not become important yet," but will be added in around these as needed. These realms, though connected, are all distant lands, and travel between them is done via map montage. ;)