more than just looking at each setting individually. The points of overlap can sometimes also be interesting, and I can think of some more game-able stuff that'll apply to these. Which I realize may seem to run counter to what I said previously.
In a sense, wouldn't the best way to satisfy our goal of making each setting feel really, really distinct be to downplay what they have in common? Like, try to not step on each others' toes if you can help it. Especially for those points that technically overlap. Yes, all three of these settings has furry races. But Mystara is the one that really leans into it, so if you're doing one of the other two settings then maybe you should go out of your way to avoid furry races so as not to steal Mystara's thunder.
But any way in which they can be made distinct from the rules of default D&D is an opportunity to capitalize on.
Here's an example. One of the more popular houserules among the cool kids is to bring back race-as-class. This used to be the standard in older editions, but it's fallen out of favor. There are many pros and cons, but the cons are so important to 5th Edition D&D that houseruling race-as-class back into play would shake things up in a huge way. Any modern player looking for a significant departure from the "default" experience will definitely get that out of this mod, and a ton of players who have grown really comfortable with the vanilla version would balk at the mere idea of such a dramatic change. And I won't lie: the downsides are very real. People like character customization for a good reason. They want options. By using race-as-class, you are simply taking those away.
But what better way to make the setting feel distinct than to create a ton of setting-relevant byproducts of a major rule change like this? That's really all race-as-class does: it changes the dynamics of the world and its societies. It changes demographics. It changes the expectations people have. It changes how you perceive different characters by placing them into more consistent roles. And it forces you to redefine what you think of each race as, since no dwarf can be a wizard and no elf can be a paladin anymore. If you go so far as to make each race into a class, then you'll also never again have a dwarf fighter. You'll have something different from a fighter, and you'll be forced to understand dwarves in their own terms, as something new.
So I think race-as-class would be a PERFECT rule for both Mystara and Greyhawk campaigns to adopt. It already fits with the in-world dynamics of both of those settings and would make any campaign set in those worlds be very memorable.
Related, we could revise the list of classes available based on each setting. We already know that some of them are setting-specific, like how Artificers are really an Eberron thing and any Psion is really meant for Dark Sun. The Sword Coast Adventurer's Guide added a fighter archetype called the Purple Dragon Knight and a wizard archetype called the Bladesinger for FR adventures and I think that's great. I may even go so far as to enforce those options if you really want to put emphasis on the setting. As in, "anyone who wants to play as a fighter will have to be a Purple Dragon Knight." Or a different approach, saying something like, "whatever the group composition ends up looking like, I want to see at least one Purple Dragon Knight or Bladesinger in the team. Figure it out among yourselves, anyone else can be whatever they want, but I need at least one of those."
Mystara maybe asks that you reevaluate the relationship between Clerics and Warlocks, as I described in this article. If there are no gods, but instead only Immortals, then maybe the key to figuring out how exactly this changes divine magic lies somewhere in how you change these classes, or even remove one.
The only thing really jumping out at me for Greyhawk would be that a normal Oath of Devotion Paladin feels a bit too pure. Maybe only Oath of Vengeance is allowed. Probably also the Oath of Conquest. Might have to drop monks if you feel they're too Asian for Greyhawk, whereas these other two settings have plenty of room for that.
Plenty of other houserules can be incorporated on a case-by-case basis for these. I think that Greyhawk thematically justifies Gold-as-XP exceptionally well, and maybe you'd even want to consider doing character creation through a Dungeon Crawl Classics-style "Funnel" adventure. Hell, maybe even use the "Gritty Realism" resting rules from page 267 of the Dungeon Master's Guide. You'll find no shortage of die-hard fans who recommend it as the single best houserule for 5E. Conversely, from the same page, you might want to try using the "Epic Heroism" resting rules in a FR game from day 1 and in a Mystara game after the PCs reach a certain level (one way to become near-Immortal).
All three of these settings could benefit from some mass combat rules if you want to feature a war. Of course, there are approximately one goddamn billion options for that if you'd like. But the point is that all three lend themselves well to that type of story, even if only for one battle in the campaign.
FR would benefit the most from a lot of city play. Some kind of procedure for urban-crawls would be appropriate, but I think the other two settings would do better with wave-crawl adventuring. While FR is famous for Neverwinter, Baldur's Gate, and Waterdeep, Greyhawk and Mystara are famous for their weird countries and far-off lands to visit.
You see where I'm going with these? None of these are "default" gameplay styles and oftentimes require some heavy lifting by the DM to implement, but they can each help bring to life that setting to its full potential. You wouldn't play a Spelljammer campaign just to do some dungeoncrawling, you'd want fucking space battles with your Treasure Planet fleet. If you're gunna do Mystara, you'd be missing out if you didn't involve an airship for the party to use.
Now, there's one more thing they all have in common which is... distinct. Remember what we put a pin in earlier?
The Earth (you know, the one we live on)Yeah, for some reason, all three of these have a particular relationship with our very own reality that one might consider a departure from the base assumption of a D&D campaign setting. That is to say, the base assumption is usually, "no relationship at all," and the very fact that something exists connecting our world to these is a bit strange. I don't believe the same is true of the other official campaign settings, except I suppose technically for Planescape.
- Mystara's map is adapted from Pangaea, the ancient super continent, implying that the setting takes place in Earth's past.
- Greyhawk once took place on a map of the United States, implying a post-apocalyptic setting that takes place in Earth's future.
- Alternatively, there was at one point a weird bit of obscure Greyhawk lore that said Oerth is just one of several "alternate Earths." The others were named "Aerth," "Uerth," and "Yarth," with varying levels of magic present.
- Even weirder, one of the human ethnic groups, the Rhennee, might literally be Romani gypsies. Like, this isn't simply a case of "base your fantasy race on a real world culture" like the other human groups. The Rhennee originally came to the Flaeness from someplace called "Rhop," which is nowhere to be found on the entire planet and quite likely involved ancient teleportation from another plane, i.e. OUR plane of existence ("Rhop" = Europe).
- And the one actually most interesting to me, and the most disappointing to see unexplored, is Forgotten Realms. The setting takes its name from a concept that featured great prevalence in Ed Greenwood's original version but is almost non-existent in the WotC version: FR is literally the world of all realms once part of our Earth but now forgotten. You might call it an "alternative past" to Earth's own history, like they were once on the same timeline but at some point it diverged into two directions, each their own planet, one magic and one mundane. Thus, some of the most ancient of all real world cultures are present in FR, such as Egyptian and Sumerian gods. Things that were a part of culture before the divergence. Isn't that fucking cool?
Sadly, this isn't really properly capitalized on in any of the three settings. At the same time, I'd be hesitant to embrace it for any of them, let alone all of them. It feels a bit... gimmicky? It's like when someone suggests to play a D&D campaign where "everyone plays themselves as a D&D character" or "themselves, but sucked into the D&D world," or someone suggests using the Dungeon Master themself as a warlock patron or, like, a cleric PC worshiping the player controlling them as their own god. That kind of stuff is always really cheesy.
The reason that such a rare trope is somehow present in all three of the classic "vanilla" settings is because it actually used to be a vanilla trope itself. For one thing, nearly all fantasy stories from throughout history just took place on Earth, because a lot of them weren't thought of as "fantasy" back then. The ancient Greeks may not have necessarily believed in Homer's version of The Odyssey, but they DID believe that those things were possible, those creatures were real, that a spell could be cast to speak with the shades of the underworld, etc. and the same being true for many medieval tales. When it comes to fairy tales, it's a bit harder to claim that the people telling them (much closer to our own point in time) actually believed in things like fairies and witches. But most of those stories were told more as tradition, something that everyone was told as a kid. Most of them belonged to traditions that can trace back to a point in time when people did believe in the fantastic elements within. Stories where the author is known to have just been making fantasy shit up are often also works where the truth of those fantasy elements simply does not matter in the slightest, e.g. allegories, political satires, social commentaries, etc. Works like Jonathan Swift's Gulliver's Travels, Dante's Divine Comedy, the ancient Assyrian/Roman Lucian's A True Story (yes, the title is being cheeky on purpose), and Johannes Kepler's Somnium (isn't it fucking wild that one of history's most important astronomers also wrote the first ever science fiction novel??).
Creating entire secondary worlds for stories to take place in is an extremely modern thing, really only fitting into the second half of the 20th century. Things that would get close to that always pulled back from committing. Alice in Wonderland was "all just a dream." So was the film of The Wizard of Oz. Even the books, which did not treat Oz as a dreamland, eventually solidify it more and more as a land accessible here on our very own Earth, an island you can sail to just west of California. Conan the Barbarian lives on an ancient version of Earth, a setting called the "Hyborean Age" that commits much more to the premise than Mystara or Forgotten Realms do. The author William Morris is credited with first trying out the "fantasy world" idea instead of a mere dreamland or "foreign country" thing. He was kind of ahead of his time, though. It took a long, long time for that to catch on. Most people attribute the shift to Tolkien and Middle-Earth. While it's true that Middle-Earth is the primary factor that inspired people to use secondary worlds, it's also noteworthy for also not actually breaking the trend.
Where do you think Middle-Earth gets its name from? Spoiler alert: it has the same relationship to real-life Earth as Forgotten Realms is supposed to (Ed Greenwood has stolen a lot from Tolkien). I mean, sort of. I'll let the man speak for himself here:
During an interview in January 1971, when asked whether the stories take place in a different era, Tolkien stated, "No ... at a different stage of imagination, yes." Speaking of Midgard and Middle-earth, he said: "Oh yes, they're the same word. Most people have made this mistake of thinking Middle-earth is a particular kind of earth or is another planet of the science fiction sort but it's just an old fashioned word for this world we live in, as imagined surrounded by the Ocean."
It's become the legendary and untouchable perfect secondary world, and yet it technically isn't. It's still doing that old trope of "all the fantasy is really just a part of our world somehow." Don't forget that Greyhawk, Mystara, and the Forgotten Realms were all created in the 70s and 80s, back when this trope was still common enough to be at least a bit vanilla.
Since it's not as common a trope anymore, maybe you could do something with it if you were running a campaign in any of these three settings. It could be a pretty novel sort of experience. I just... I don't really have any advice for that kind of thing. It seems a bit silly to me. But I have a feeling that, if you pull it off well, then it would go a long way in accomplishing the goal of this series: make the players remember and feel a setting that, on the surface, looks unremarkable.