Does there exist a setting which is so unique that it would require a bespoke system to run it? Do you know of any settings across fiction?

Does my own setting count? I'll detail it below.

My setting operates off of the assumption that the world is a composite of three semiotic forces. Narrative, Symbol, and Belief. Therefore, tone, theme, and plot are real objects in the ontology of this world, meaning a character who is sufficiently awakened to the nature of the world can gain powers like 'One Punch Man', the power to kill anything in one hit no matter what.

My guess is probably not. 

Unique settings likely need to have special rules for the unusual bits, but most of the time the regular rules from your favorite game will do.

Personally, I think the vast majority of settings that weren't actually designed for an existing RPG would benefit from having their own bespoke systems. But there's a high level of variance, both between settings and more importantly, in people's sensitivity to the discrepancies. There are some people out there for whom their preferred edition of D&D is going to be "good enough" to run literally any setting. They just aren't bothered about it. Then there's people like me for whom any discrepancy between the game system and the setting is going to chip away at their immersion. As a matter of practicality, I acknowledge that I don't have time to do a homebrew hack for every campaign, but I spend a lot of energy browsing multiple games to try and find the best possible fit for any setting I consider running.

IMO every setting should have its own rules. However, more important than the features of the setting, the tone or genre of the setting needs its own rules.

That's why the glut of OGL/d20 products that came out post 3E all felt generic and forgettable, despite covering a vast range of setting concepts and genres.

Does EVERY setting need a bespoke system? No.

But it does need a system that plays to the right genre expectations and not all systems do all genres equally well.

For Mecha action, my goto is Silhouette (Jovian Chronicles/Heavy Gear) with a dash of Mekton building rules on top.

For Urban Fantasy settings I default to my "White Book Mage" (a ground up rebuild of OWoD's mechanics I did after the line was ganked and access to the books started becoming difficult for my players) that I've since generic'd enough to be useable for a lot of modern settings.

For Pulp Adventure my default these days is Savage Worlds; though I'm going to get a chance to try FATE soon so we'll see how that goes.

For fantasy (and terrestrial science fantasy) my default is my Ruins & Realms system.

For space opera it's WEG Star Wars with some house rules (mostly going counting successes instead of summing dice and splitting out character points into separate XP and burn to improve rolls pools).

I actually use WEG for Star Trek as well, but with different attributes (only one for physical tasks, but five mental ones plus a possible PSI stat for species that need it to reflect the setting's focus on reasoning over physical acts in problem solving) with the skills reorganized to fit them.

And lastly M&M for superhero and related shenanigans (particularly action heavy urban fantasy settings).

It's all about finding the system that aligns best with the genre.

It depends on what the game is trying to accomplish. If you're writing a game about vampires struggling to maintain their humanity, then it makes sense to design the system from the ground up to handle that rather than crudely tacking on a humanity statistic to an otherwise generic system.

Dragonlance may have started with AD&D, but I liked it better with its own bespoke system. (And I like that system just as well or better for Ravenloft, too. )

Quote from: Chris24601 on March 30, 2025, 12:24:47 PMFor Pulp Adventure my default these days is Savage Worlds; though I'm going to get a chance to try FATE soon so we'll see how that goes.

When my group wanted to do swords and sorcery in the style of Fritz Leiber's Fafhrd and the Gray Mouser series, we went with Fate Accelerated Edition and had a blast.  Fate points are a great tool for that kind of pulp action and the system rewards you for creating advantages for your team mates or hindering your foes.

My own system mostly falls into this category. I don't talk about it that often here because this is basically an OSR crowd and I don't do OSR, but the entire premise of Selection: Roleplay Evolved is that the campaign's antagonist and the players get locked into a headgame mediated through monster design to create the feeling of an evolutionary arms race.

Specifically, the players can veto the GM from using monsters with a set ability for a session, which in turn encourages the antagonist to design monsters around that limitation. The entire point of this process is to make it so players are deeply aware of monster design and it doesn't feel like the monster design falls into ruts unless the GM is doing that on purpose.

This naturally necessitates a significant amount of GM prep, but it also basically doesn't work without the system attached to it. In theory you can use another system and scratch out abilities, but in practice most systems will just use a different premade monster template. This will not quite feel the same in actual play because the design overlap doesn't feel as organic.