make the whole continent of Tamriel, and before anyone jumps on me about this, hear me out first.
I know some people are concerned that Bethesda would just cookie cutter locations, but I have to disagree with that, since Bethesda has always made an effort to make every location unique, the only problem was the NPC's, which they did fix in Skyrim.
They don't need to make all of Tamriel availible right off, possibly just Cyrodil and one or two lands that already have basic models for Morrowind, Oblivion and Skyrim.
Make those three availible right off, just upgrade Morrowind and Oblivion with Skyrim's graphics and gameplay mechanics, and make them instanced, so that when you leave Cyrodil for instance to go to Skyrim, a few days or weeks pass in-game time to arrive there.
Also, make it possible to choose where you start, such as in Skyrim, Morrowind or Cyrodil.
Make Elswyr and the other lands availible as DLC content once they are created, this will have several advantage's. The first being that it maximizes the use of the graphics and personnel needed for the project.
The second would be that this will generate additional income for Bathesda, as well as keeping things interesting.
Make the Akaviri lands availible later on as well, which would add Akaviri items, weapons and armor to the game.
I would love to see it take place just after Skyrim, only this time, your another Dragonborn consolidating power to take on the weak, and corrupt Empire and the Aldmeri Dominion, and the goal is to gather as many allies as possible.