this post by Ryan M. Danks just now that talks about how to get around the conundrum of writing Settings for the many RPG Systems out there. It was a thoughtful post, and provides some interesting advice. But the more I thought about it the harder I thought it would be to actually follow the advice. Once again, the problem is that ... it's complicated.
The advice seemed pretty spot on advice to me at first, in that Settings are the gift that keeps on giving, and frankly I don't think it is possible to saturate the market as the options are infinite.
As I see it RPG Systems tend to be rigid and can force Settings down certain paths due to the intricacies of the rules. For example, most systems come with specific items, classes, spells, and so on, specific in many cases to that particular system. The implications on the world are huge, and can limit you into making Settings choices that aren't really what you wanted to write about. But since, for example, the rules have Fire Ball spells, you kinda have no choice but to take them into account. Otherwise the GMs who buy your setting for System X will complain that there's no Fire Balls, and everyone knows the Heroes are going to have Fire Balls. So you include Fire Balls as one of the Spells for your new evil Shamen class. It doesn't fit, but hell with it. The System has Fire Balls, and so you're obliged to factor that in. When you play to a System it shapes your Setting. It has to. In a sense, that is what Systems are for.
However, if you write Settings that don't include anything from a known setting, but instead includes things you have in mind that are different and cool, such as a handful of spells that actually do fit your new evil Shamen class... well, now you kind of have to define those spells and how they work so that once they bought your Setting the GMs and Players know what to do with them. But then once you do that you're kind of on rocky terrain because that suggests that the spells you want to create for your Setting are going to have some System stuff associated to them. The evil Shamen spell "Shadow Fist", for example, might be defined in your Setting as doing 1d6 damage to the recipient's Mystic Points. But what if the system the GM likes to play doesn't have Mystic Points, but uses some totally different mechanic for magic? The GM will then need to translate into their preferred System. It gets messy. You could just say, instead, "Shadow Fist is a spell that does spiritual damage," but does that really solve the problem? The GM and the Players may wind up scratching their heads and asking "Well, how do we account for that?"
I think a possible solution is to write stories. Stories that are representative of the Setting you have in mind. And include in the stories history, main characters, and descriptions of the kinds of things that can happen. Include maps. Include a plot line. But whatever you do, don't include specifics, like armors, weapons, spells and whatnot. Adding any of that would open the can of worms. Then you market the Story as a Setting for RPGs, just with the technical details left out. This would allow GMs for any system to pick up your story and run with it. Maybe.
To be honest, I'm not sure how many GMs will want to purchase Settings if they don't come with the technical stats that they need to run the game. A lot of GMs are perfectly fine with creating their own histories, NPCs, plot lines and so forth. Where they need help is in saving time creating the Stats for all their monsters, villains, NPC heroes, and treasures (that fit their world), and so on. GMs will gravitate towards anything that is creative AND saves them time.
Unfortunately, as many people have pointed out, there's a huge glut of RPG Systems. And there's more every month. Everyone wants to create Systems, and tie them to their Settings. In fact, I believe that has been a mantra of the Indie Revolution for some time. The Rules must actively support and reinforce the Setting. Of course not everyone is writing Indie RPGs and there's been a big push among OSR enthusiasts to also produce RPGs. But even so, it seems to be that Settings-Systems come together and are fused into one thing. And consequently it's often hard for GMs to take Settings materials that were written for one System and apply them easily and efficiently to another. They are still stuck translating from one System to another, and it is still time consuming. And there is always the matter of Stuff that's in a Setting for System X that doesn't exist in System Y.
It may be there is no easy solution. We may have to go back in time and explain to Gygax and Arneson that they need to focus on simplifying the rules rather than expanding them, and focusing on Settings that can fit into those rules easily, thus making the market about Settings, not rules. Unfortunately that didn't happen. So here we are. I'm not sure there's a good solution to the problem at this point. But if there is any, it's in a simple system that allows conversions to other systems as easily as possible.