AdmundfortGeographer
Getting lost in fantasy maps
Mystara?Meanwhile, Darksun/Mystara/Dragonlancece/Greyhawk (...) don't seem to mesh with something else than a prime material plane.

Mystara fits better than any setting with the 4e cosmology.
The description of the domains of dread (yeah, I know, clearly Ravenloft) also leave room for Castle Amber.
The Astral Sea? The description sounds exactly like how the Astral Plane worked in Mystara all these years. Each outer plane floating in an endless expanse.
Mystara does have the fairy realm The Dreamland, and its surrounding forests, this could easily be a piece of a larger plane of Feywild.
Mystara's abundance of dimensions leaves more than enough room for the Shadowfell.
The flavor change in the elemental planes is not that big of a deal, and plugs right in to Mystara's elemental planes anyway, IMO. As I said, Mystara fits so well with the 4e cosmology that I would be surprised to learn that the 4e designers went back to the BECMI/Mystara line and mined it for ideas liberally.