I don't have anything to back this up, but maybe that's why some of the plug-ins have been sub-par. I think that's where their attention is focused. It would be worth $20 to me to have a Hammerfell or Summerset Isle addon.
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[FONT=Times][I][COLOR=orange] Nil Mortify sine Lucre
Wurlon makes a good point ... making an expansion for a whole very populated province would be just too hard to do it justice. But there are less populated or at least, less technically built up provinces .. so maybe Black Marsh or Valenwood could be a possibility for an expansion. Kind of similar to Solstheim which I know was just an island but it was very underpopulated. I liked the expansive feel of it.
Or they could do an expansion of just one city in another province to give us the feel of that province. I think picking central Cyrodiil was a smart move if they are going to connect any provinces by expansions in any way.