Instead of focusing on the setting first, I instead made a list of requirements for a game I wanted to run. Each, in turn, has consequences for the setting:
This is highest on my list of requirements. To be able to do preparation for gaming expeditiously is a necessary or there will be no game at all. Despite this…
…I don't want to use a commercially-available setting. I don't mind liberally borrowing or adapting things, but the cognitive load of keeping the details of someone else's work is often a lot higher than just doing your own.
Much of the old pulp literature that OD&D was based on was written with the merest sketch of characters, locations and situations; as they wrote throwaway ideas were incorporated for depth and potential anchors for future growth if the stories become the root of a larger world. Gaming can be similar.
I want the game to be available and approachable by people who want to play, be that every session, when their schedule allows or someone who just wants to try it out. That means that to facilitate engagement, the base setting needs to be closer to a fantasy lingua franca to get people immersed in the game more easily.
I want a game that has a West Marches style play. I want players to be able to do the scheduling and control where they go, plan their own missions and chart their own courses. I want information sharing. I want competing playgroups and friendly rivalries. I want to be able to see the results of multiple groups changing the world.
Finally, I don't want the game to be too serious. No big plot, just some locations, some antagonists with some goals, some situations, and let it evolve. Gameplay can focus on that, or it can just be about kicking in doors and taking monsters' stuff, it's all good.
From this I could basically say the setting is designed around these ideas as its touchstones:
A sketchy homebrew world littered with the stubs of ideas can grow as needed and still be rich and engaging. This thrives on the prepping of locations, not plots.
A sandbox-style game that is set up for such that players can drop-in or come and go as needed will build a larger player base or lend itself to more situations where the game can played. If players find a plot hook they want to pursue, then they are welcome to drive the direction of the game.
If you want new and occasional casual players to be oriented, then you need a common frame of reference. Almost everyone nowadays understands the idea of “D&D fantasy” to some degree, and gives them a familiar place to start, even if the similarity may only be skin deep; at least the really fantastic stuff will seem like actual fantasy then!
It's a game. Let's have fun and let the game world reveal itself through play itself. It can be deep or shallow, based on the desires of the players.