Well, as I mentioned in another thread, I had a "flying island" setting I thought briefly about submitting. Decided not to for various reasons.
Number one is that I just don't see it as a game setting. It feels like a fiction setting to me, if that makes any sense. I've already written two novels in this setting, although neither is published yet.
Two, those novels are already copyrighted and on file at the Library of Congress, and I have no idea how that would impact WotC's guidelines/legal rules. What can I say, I was paranoid at the time I wrote these novels.
Three, I felt it was too complex to make a good setting.
"Too complex"? What does that mean?
Well, in this setting--called CloudSea--each of the larger (continent-sized) islands is based off a mytho-historical Earth culture. Pirene is Imperial Rome. Lucinde is Chivalric England (a la King Arthur). Achaea is classical Greece. Cheth is tribal Africa. And so on and so forth. Between the various human kingdoms, and the various elves, dwarven, and goblin kingdoms, we're talking over a dozen fully developed cultures, each of which could require a sourcebook in its own right. Now, some would say that this requirement makes it perfect for a product line, but I felt that I couldn't do it justice in a core book of its own.
Mostly, though, as I said, it just didn't feel right for this open call. And since I had several other ideas that did, I decided against this one.