I will not tread heavily into summarizing the well-known principle of the
"uncanny valley" (as per the link) regarding the corollary relationship between an object's resemblance to a human being and the emotional response to it. But I am wondering whether Fantasy TTRPG settings operate as the inverse. This is to hypothesize that there is a general "canny valley" of psychologically acceptable play with settings for the aggregate of people between the "all too historical" and "all too ahistorical."
It is difficult for people to relate well to both the more historically accurate societies and the more utterly fantastical ones, both being "alien" or "uncanny" in some regards to cultural mindset of players of contemporary society, particularly the greater the amount of detail and depth these settings are given. On one end, the settings are perhaps too similar to the familiar, while on the other end, the settings are to dissimilar to the familiar. So settings often have the onerous task of striking the right balance between the poles of familiarity to create a "canny valley" of play. Outside of this "canny valley," players have difficulty psychologically plugging themselves into the setting and so such settings are mostly niche. Examples of possible niche settings may include settings like Hârn and Tékumel.
But it's also possible that we are not dealing with a canny valley of playable settings at all, but, rather, we are in fact dealing with an uncanny valley of unplayable/niche settings.
I think this is largely true, as an approach. The settings I've seen players engage with most tend to be either:
1) Set somewhere from about 1860 to the near future, with some supernatural, superhero, or sci-fi elements.
or
2) Set in a totally unrealistic past/future that somehow has societies that aren't particularly fantastical, but actually kind of straightforward/modern-day, just transposed to use medieval or futuristic tropes. Basically
Disenchantment and
Futurama, in a sense. Star Wars is a good example of this, where, despite it being a galaxy far, far away, long, long ago, it's basically the 20th century in SPAAAAAAAACE in a lot of ways, with fascists, rebels, unscrupulous capitalists, and so on.
And yeah things like Harn, RuneQuest, Eclipse Phase and so on challenge this, by not conforming, and are rewarded for their bravery and often excellence with mediocre sales and limited popularity. Glorantha is awesome but it definitely crosses some threshold that makes it challenging for a lot of people to engage with.
Basically if you game isn't largely 1850-present in terms of mores, social concepts, and so on, you are limiting the audience, perhaps steeply. It's fine to have a setting where you juxtapose something alien next to that, that can work, of course. But if you go full historical realism, or full fantasy, or even fully-realized sci-fi, a lot of people will be turned off.
I think one successful approach to this sort of weirder setting might be to make the default PCs be outsiders to the setting. I.e. if you have some wild/wacky sci-fi setting of a highly speculative nature, where it might be hard to come up with an "in-setting" PC for a lot of players, just have the default be that players are all 21st century people who uploaded their brains in like 2050, and have been downloaded into cloned bodies and so on, so the players can experience the setting as an outsider.
It's important that this doesn't limit what class-equivalents or power-sets the players can access though.
This is a good thread because it makes me think hard about what I should actually do if I create a new setting/RPG.