It's probably too late to add these areas in the main game anyway, but I was thinking: The Main attraction of TESO is that, for the first time since Arena, we are able to explore the entire continent of Tamriel, but why stop there, why not branch out into unknown territory that we have only read about in the in game books? It would add more territory to explore for all factions, provide some exciting end-game content (which, from what I understand, can make or break an MMO), and allow Zenimax to flex their creative muscles.
These would mostly be PVP neutral territories, where people of different factions are allowed to fight each other, but they aren't in direct competition with each other like Cyrodiil.
Akavir: This is probably the most well known of the provinces I will mention, I also think there is great potential here, one atheistically because the asian/roman style is rather unique, but two because there is a lot of infighting between the four nations and it could be a great backstory for questing or raids. (PS, this would be about four provinces in size, the others would be one)
Pyandonea: Again, the Maomer would make a powerful enemy. Maybe there would be a reason for the armies who want to invade here, and the players capture forts and defend against large scale attacks like how PVP in Cyrodiil is supposed to be, except it would be against NPCs, which might be cool for people who don't like PVP
Atmora: Not much is known here, so there could be a lot of room for Zenimax to come up with cool stuff to explore
Yokuda: Before you say it: Yes, I know Yokuda sunk, but I was thinking, maybe there could be organized underwater expeditions to reclaim lost riches. This would make the argonian bonus feel not so useless. and add some variety to the combat.
I don't know, what do you guys think
PS: I think Solstiem and Stros M'kai should also be included as part of EbonHeart and Daggerfall territory respecitvley. And then for DLC we could visit daedric realms