Thoughts on a possible future project

Like a lot of writers, I'm usually working on a couple of things at any one point and my brain keeps trying to throw other stuff at me because it's always exciting to work on (or at least consider new ideas). And there's one that's been popping up on my mental rotation for a while that I'm giving some serious thought towards, perhaps when I've got a few things off my plate. I don't want to go into too much detail because:

1. I haven't decided for sure if I'm going to do it and don't want to make promises I might not be able to keep.
2. Even if I do it, I can't make any promises as to when I'd get it started and I'd rather not encourage anyone to ask me how it's coming along until it actually starts, y'know, coming along.
3. Talking about stuff like this too much can cause you to lose enthusiasm because your brain gets the same amount of satisfaction (there are actual studies on this).

But the relevant info (and the reason for this post) is this: The project in question is a small RPG supplement, likely using Fate Core or Condensed. But what I'm unsure on is the setting material. It'd be an original setting, and while I have a pretty good picture of it all in my head, I'm not sure what needs to go into the book. I mean, I could probably write up the basic pitch and character creation guidelines and call it good, but this is something I'd actually like to put up somewhere (probably DriveThruRPG) and sell, and I can't justify going to the trouble just to make what would amount to a pamphlet. In terms of size and quality, I'm shooting for something on par with Evil Hat's Fate Worlds line.

So my question is this:

What do people think would be essential for a basic glimpse at a setting, something that could be used for people to run games with just the one book without that book needing to be huge? What would you want to see in a basic setting supplement?

I've got plenty of books both big and small, physical and PDF, and I could easily just study any or all of those, but I've got so much to look at I don't know where to start, and that's what I'm looking for now. What do you all think would be useful starting points?

Also, in doing a bit of research while writing this post, I've discovered there's at least one book out there that (going by the blurb) already seems to come pretty close to what I've got in mind. So I guess I've got a second question:

Is there any point in me going to the trouble of writing my own supplement and asking people to spend money on it when it'd probably be about as productive to just write up a blog post and point it at the other book?

I mean, yes, I guess at this point I'm blatantly fishing for encouragement, but I think I've reached the point where I'm just going to need that nudge. It's not something I do often, it's not something I want to do at all, but to be totally honest I'm discouraged enough that it's all I can do to even finish this blog post rather than just say 'screw it' and delete the whole thing.

Sorry, that went into a tangent I'm not proud of. A lot of little frustrations are getting to me.

If you're still reading this, I'd appreciate feedback either in the comments here or in a reply to wherever you initially read this. Thanks for your time and your help.