As I posted in this thread
, I have seen speculation that this new setting might actually be Mystara, in the form of the Isle of Dread reprint. It makes a lot of sense given the quote "a book that we have not created is for a new setting" since the Isle of Dread reprint will be published by Goodman Games.
Now in this thread
we speculated that the Known World information might be purged from the Goodman Games release. Perhaps this is confirmation that this is not the case and that WotC will indeed allow GMG to pubish the full content of both versions of the Isle of Dread (both containing an overview of the KW nations of Mystara)?
OTOH, if it is as simple as that, why wouldn't Nathan Stewart simply say so? Well, he is the guy who tweeted the image trolling people into thinking they were doing Spelljammer only to reveal that the title said "summer" so maybe that's just the kind of guy he is?