Nah, it wouldn't fit in the Shadowfell for numerous reasons. For one, the Shadowfell is a reflection of a specific world. Faerun has its Shadowfell reflection, Krynn has its Shadowfell reflection, etc. Ravenloft isn't a reflection of any world. There're lands torn or copied from other worlds on it, but it isn't a copy of any one world. Second, it's a whole lot easier to get out of the Shadowfell than it is to get out of Ravenloft. Last but not least, it's a demiplane (which still exist in
4E), and as such, would probably be situated in the Astral Sea. Besides, it'd be nice to have Ravenloft with its own Shadowfell and Feywild reflections.
As for this news, personally I'm sweating bullets at the thought of making it "core". They went to the trouble of keeping demiplanes in
4E, so hopefully they'll keep it as one. One which is insanely difficult to get out of. And hopefully they won't feel the need to force everything in core into Ravenloft. Namely, Dragonborn, Dragons, planar travel, etc. Those're pretty big defining characteristics of Ravenloft, and I'd hate to see it lose them.
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