What do you see as the issue with Ravenloft being located in the Shadowfell of a demi-plane of the Shadowfell? What are the negatives? Any positives?
I don't think there are any problems, except for
myself because in the past I have envisioned and used a very different Plane of Shadow, and I would like to keep using that.
I still have the "dark and twisted counterpart of the material plane" concept in place, but I have always strongly emphasized
shadows as the key feature of the plane, particularly the fact that everything and everyone on the Plane of Shadow corresponds to something and someone on the material, albeit with critical differences (e.g. a huge forest on the material could be a tiny garden on the shadowplane and viceversa -> this lays the fundamentals of using the shadowplane as a shortcut, but with a danger of actually getting the opposite effect). You know when as a child you were falling asleep in your bedroom and thought the cloth hanger looked liked a monster in the dark? Maybe that's because it
is a monster in the plane of shadow!
As for creatures, I've typically had the 1-to-1 symmetry imply that the number of shadow creatures is the same as the number of creatures on the material, and each material creature has a shadow double. Among other things, it means each time the PCs use the shadowplane as a mean for travelling, their corresponding shadow creatures are transported into the material and wreck havoc (a good way to contribute to the dark reputation of shadow magic), although this was always kept more like a legend than actual fact.
These can still work for the official Shadowfell. What doesn't work well enough for me is the undead, as my shadowplane creatures have always been something different that is assumed to exist simultaneously with their material counterpart.
That said, it's not completely incompatible. It could even be interesting to drop the ball that Strahd must then have a material counterpart, but who could it be? Or is he just an exception, did he carve out his dread domain within the Shadowfell managing to evade its rules?