I'll just repost what I posted in the comments:
I certainly in support of Planescape as the default assumption of the D&D Next cosmology. The Great Wheel was a very flexible cosmology, and even in Planescape which assumed a bunch of truths on many topics, also brought up many mysteries and uncertainties on other subjects. And philosophically in terms of Planescape itself, one could simply say that the Great Wheel is a theory on how the multiverse is mapped, one of many theories, but the most popular and most proven theory.
I'm actually alright about the concept of border elemental planes, but I feel there needs to be something to conceptually distinguish the Elemental Chaos from the Outer Plane of Limbo which 4e tried to throw in a blender with other places. Feywild was an element of 4e I liked, but I'm skeptical making Ravenloft the border between Negative Energy and many Prime Material worlds. I know a lot of reconceptializing to Ravenloft which to me is a demiplane with a continent containing Barovia, Darkon, Falkovnia and other places surrounded by "the mists". Simply put, it might be better to use Shadowfell as the border concept instead.
On the subject of Spelljammer I never liked how they tried too hard to fit the other campaign settings in, they got as far as FR, GH and DL, and every other setting they came up with excuses for why it doesn't fit. Things like the campaign homebase the Rock of Bral which I always insisted existed in it's own sphere independent of the other settings. Spelljammer I felt was at it's best when it was more about strange worlds and ancient spacefaring civilizations.