In fact it would be the still 'whole pie'. I am not talking about making uncomplete games. Think about several devs developing new content and adding it to Oblivion once in 6 months... Or one year. Every add-on will be like a new game. I saw that 'elsweyr' mod and I was really stunned. And as you said, they are just modders doing the work free... Think about professionals doing the same thing. It is like adding new games to the older one. And this way it will be a whole story after all provinces are added.
The elsweyr mod was all done with one person, within one/two year with alot of modder resources use up, and was able to get it updated when the time is needed. All this with the mesh, texture, animation, script, and what nots the game already provided for the modder to build on for the elsweyr mod. I rather doubt the professional have that much time to do one addon for that if they have other projects and publishing to worry about. Beside, I rather if they do a project, they do not just rush it within a 6 month - year bases but rather in years (3-6 years in fact) to make it decent.
And that still a "piece" of pie if ya do it that way. It does not do any justice to put the province as "addon" if all the culture, mission, quest, people, mobs, texture, meshes, weapons, leveling scale, and etc to be smush up to an addon to a continuing game and all done with in a short amount of time. If they going to make a province addon that takes a long time build (like 2 - 6 years), they might as well make a whole game base on it and that right there is a "whole" pie.
I dont mind if graphics are better or not because the story and deepness of the game is more important for me. Sometimes imagination does a better work than a graphics card. That's why there are lots of people who still like morrowind more than oblivion. On the other side positive changes are still important about a game and if they can be added with the new expansions, that would be awesome.
And I did not talk about graphics. There is a hell lot reason why people will choose Morrowind over Oblivion, and its not just about graphics (in fact with a certain program, Morrowind's graphics is equal or superior to Oblivion's). If they are going to add in expansions, I rather its done in a land base with content, like that from Morrowind's or Oblivion's, and not make it like a whole province (its too big for that) or small stuff like that of DLC from Fallout 3.