“Syncretism” is the religious concept that two or more previously separate religions were secretly the same all along. Sometimes this is presented as “both religions contained part of the whole truth” and sometimes this is presented as “your religion is just my religion wearing a funny hat.” We’re not actually talking about religions, here, we’re talking about fictional settings (anyone who thinks adding a “what’s even the difference” joke is still edgy will be flogged), which means we are focusing mainly on that first one, the challenges of combining different settings together.
This comes up a lot in free form roleplay. All my examples are ten years out of date because I last did this kind of thing in high school, but while the specific settings that are popular have changed and the medium for creation has almost certainly moved away from the forums and chat rooms I used as a teenager, I expect the basic principles still apply to babby’s first crossover setting even today. Back before I reached my elderly twenties, the most common crossover setting was Kingdom Hearts. It was popular amongst the kinds of nerds who did freeform roleplay in the first place and had what appeared to be a built-in means of stitching any number of settings together. One example I remember in particular involved someone making a map of an expanded gummi map that included four different worlds from Star Wars (I remember Bespin and I think Coruscant, but the details don’t matter).
I bring up this example because it’s a good demonstration of how not to do syncretism. Method #1: Different settings are haphazardly jammed together without any means of influencing one another, and quick excuses are used to paper over why they have completely different tone, technology and even physics from one another. I’m sure there was some explanation for why the Millennium Falcon could travel to Bespin but not to Halloween Town, even if it was just “we never explored what lay beyond these four worlds because they seemed pretty sufficient,” but the seams between worlds are extremely obvious. The Final Fantasy style fire/lightning/blizzard magic has some overlap with the Force (Force lightning is a thing), but there’s no precedent for mind tricks and telekinesis is a very high-level trick in KH, usable only temporarily as part of a special super-transformation, while it’s one of the first tricks that Jedi learn. Star Wars ships are buckets of bolts clearly distinct from gummi ships, which, as the name implies, are made of some kind of gelatinous or play-doh-like cartoon substance. The fact that the Galactic Empire has a giant army of stormtroopers and star destroyers demonstrably capable of traveling between some worlds doesn’t lead them to become an immediate and overwhelming threat to all neighboring worlds because their inability to travel past the original Star Wars worlds is handwaved away.
This method also has precedent in the “wormhole randomly opens up between Federation Space and the Galactic Empire” conceit that was popular in the 80s and 90s, and while that’s perfectly forgivable in the context of a Star Trek vs. Star Wars hypothetical situation, it’s not so forgivable in the context of actual Trek/Wars crossover fanfiction or roleplay, again because the seams are so obvious that it makes the setting very obviously artificial. It also means that the only thing you gain from the syncretism is the ability to take characters from one setting and put them in another. There’s no shared history.
Also in the 80s and 90s, D&D and Magic: the Gathering stumbled by blind luck into the far superior (but still flawed) Method #2: Link different settings that already share similar technology and physics together through any super-transit you like. In Spelljammer, you can sail your space galleon through the ether from Krynn to Toril to Oerth, and that fundamentally works because those three settings all have the same wizards, clerics, and knights as one another. None of those settings have varying magic systems that clash jarringly with one another, nor does one threaten to invade and take over the others due to drastic differences in power. Elminster, Mordenkainen, and Raistlin’s differences are mainly cultural. In Magic, everything runs on the five colors of mana, which ties the worlds together thematically and makes them fit together like the world’s most boring, perfectly modular jigsaw puzzle, and while that jigsaw puzzle is super boring to solve, it also allows unlimited storytelling potential amongst the settings that exist.
The problem is twofold. Number one, different settings still tend to interact with one another hardly at all. In Magic: the Gathering, it is rare that anyone is capable of projecting significant amounts of force from one plane to another, which makes each plane a mostly self-contained setting. Planeswalkers form a recurring cast of heroes and villains who can pull their favorite pet angels from whatever plane they want to wherever the plot is this week, but it’s pretty rare that an entire villainous faction with armies and infrastructure and such invades another plane. Likewise in Planescape, where plane shifting is so rare an ability and so limited in scope that plane shifting the dragonarmies into the Sword Coast is so impractical that it’s not surprising no one has done it. In Spelljammer, despite the fact that astral fleets do in fact exist, the different settings don’t interact with one another significantly anyway, for no better reason except that this would require significant rewrites to the settings being syncretized and the authors didn’t want to bother with doing that.
This brings us to the most difficult but also most consistently popular Method #3: The different settings are tweaked to allow for the events and elements of one to impact the other. This is the method used by comic books, including the Marvel Cinematic Universe, and is also the method used by ancient myths and legends like Greek Mythology or the Matter of Britain. Shared universe fan theories also tend to aim for this one, since the different settings actually showing up to impact one another is a prerequisite for it to really be a fan theory rather than just headcanon. The Pixar shared universe theory, for example, posits that different movies represent different points along a shared timeline going from 1992 in Toy Story to 20XX in Wall-E.
The advantage of this method is that you can take full advantage of the different settings by having the events of one actually impact the situation of another, thus making a truly shared universe, rather than simply having a handwaved explanation for why characters from one universe have showed up in another, clearly distinct universe. The lines between the different settings become indistinct. The disadvantage is that each new setting added requires exponentially more work to figure out how they impact one another, although you can cut this down to linear rather than exponential work by giving the settings different borders to one another, i.e. if you can go from Krynn to Toril and from Toril to Oerth but not from Krynn directly to Oerth, then you don’t have to worry about how Krynn affects Oerth directly, because it just doesn’t.
This means you have to come up with an actual explanation for what the Force is and why it works differently from the Final Fantasy spells used by the rest of the Kingdom Hearts setting, and instead of figuring out an excuse why the Galactic Empire doesn’t invade Halloween Town, you have to figure out what happens when they do. The advantage is that you get to tell stories about the Galactic Empire invading Halloween Town.
That particular example sounds kind of like a joke, which brings us to the most important thing that a syncretic shared universe needs to do, and that is create from the very beginning a super-setting that can successfully contain all the lesser settings. The Kingdom Hearts super-setting – the conceit of gummi ships and Mickey Mouse as god-emperor of toonkind and so forth – cannot contain Star Wars, so the Galactic Empire invading Halloween Town sounds super weird and ridiculous. Any super-setting that contains Star Wars within itself must either contain multiple galaxies or else rewrite Star Wars as taking place in a much smaller sector of space (this causes less damage than you might think – for all that “a galaxy far, far away” is iconic, even plumbing the expanded universe gets us only maybe 400-500 distinct worlds that need to be accounted for to tell every Star Wars story). If it’s going to contain both Star Wars and Kingdom Hearts, it needs to explain how a single setting can contain both Jedi and a talking cartoon duck with magic superpowers. This is not as hard as it sounds – you get used to said talking cartoon duck wizard walking around photorealistic Pirates of the Caribbean surprisingly fast – but it does represent a significant obstacle. Donald Duck must overshadow Jack Sparrow in importance, because while Donald Duck is perfectly capable of doing his bit while a bemused photorealistic Caribbean pirate looks on, Jack Sparrow cannot have a dramatic moment with a cartoon duck in the background without that moment being diminished.
As this example demonstrates, there are limits to how far syncretism can take you, but they stretch much further than you might think, especially if you are willing to reboot the involved settings to play more nicely with one another. Donald the cartoon duck cannot fit into the setting of Pirates of the Caribbean or Star Wars without damaging them. Donald the short aaracockra with a shorter temper totally can. And he can still be a sitcom character and he can still have wizard powers, so long as you do the work of explaining how Donald’s wizard powers, Barbosa’s undead pirate powers, and Star Wars’ Jedi powers are all different flavors of the same metaphysics. Bonus points for coming up with Brandon Sanderson style metaphysics that can explain without explicit statement how they would interact with each other, but a bunch of arcanobabble that puts them in the same system as one another without being able to make any predictions about how they’ll interact works just fine.
I haven’t actually done the work to make Kingdom Hearts and Star Wars play nice with each other, and so they don’t, which is why this example might still seem like a stretch. For a good reason to believe that it’s not, I refer you back to the Marvel Cinematic Universe. This is a setting that currently contains an occultist wizard, Norse gods, a wacky band of space opera misfits, a super-science mutant rage monster, a super-science suit of power armor, a super-science army of evil robots, and which could incorporate super-science spontaneous mutation from the X-Men universe with no difficulty at all if there weren’t a copyright snafu in the way.
Getting Doctor Strange to share a setting with Iron Man is not fundamentally harder than getting Luke Skywalker to share a setting with Sora (well, except in that the parallels between Luke and Sora will draw attention in a way that Strange and Stark won’t) or Elminster. That is to say, it’s very hard in that you have to redesign those characters a bit so that the seams aren’t quite as drastic while keeping them recognizable, and then you have to give them really good debut stories that puts them nominally in the same setting as one another without actually giving them a major crossover, and then you have to make another really good story where they team up to fight crime together, and then after that it will seem perfectly natural and your job is done. So, not actually easy at all, but demonstrably possible.