never.
the development time and the amount of people it would take would cost way to much money for them to ever do it. I would like it as much as the next person but its pretty much asking to lose money for Bethesda.
This.
Considering the amount of work that would go into filling the provinces -and Skyrim, Cyrodil and Morrowind do not constitue 'all of Tamriel- would mean that it would take as long as Skyrim to make, they'd have to charge as much as the main game cost and they'd have to be sure the the DLC sales would match the games sales to make the kind of money to recoup their costs.
And since Skyrim took about five years to make, the DLC would arrive around 2016. And who's going to be interested in a five year old game by then to warrent buying DLC for it that costs the same as a new game?
edit: And people are making far too much out of the landmasses being there. The same was done in Oblivion, the neighbouring provinces had basic terrain. Don't remember Province DLC for that game.