I think you're starting to be deliberately obtuse here, as all I've suggested is making Innistrad a Domain of Dread, which honestly could be thrown in as a blurb in an Innistrad book without changing anything at all about the setting. It's just a nice flavor thing that ties Innistrad closer to D&D settings, without actually changing anything important.
That's a hell of a lot different than making a combined Dominaria/Greyhawk/Mystara/FR setting. Crawford has already said that Magic settings all exist on the Prime Material Plane in D&D cosmology. All I'm suggesting is moving one of them, Innistrad, into Ravenloft (a move from the Prime Material to the Shadowfel), as Innistrad has a lot more in common with the Domains of Dread than settings on the Prime Material Plane.