1. It will be called Dominion, and be based in the heartland of the Aldmeri Dominion (Summerset Isle).
2. No matter how you play Skyrim, things turn out for the Dominion's benefit. Canonized lore will have it that the Dragonborn completed the Assassin's Guild questline (ends with the assassination of Emperor Titus Mede II), whose end results in the fragmentation and collapse of the Tamrielic Empire. The whole civil war questline ends up in the dustbin of history; in a certain sense, it does not matter whether or not the Empire or Ulfric wins, as the whole of Tamriel splits into a bevy of statelets which the Dominion then vacuums up.
3. Thalmor religious persecution has developed into full-blown genocide. And Mundus is worse off for it; in fact, it's buckling.
4. It will be the Hero's job to assume Talos' divinity and stabilize Nirn and the firmament. The second-most-important task is destabilization of the Dominion, by now roundly considered evil.
Of course, this doesn't undermine the ability to make moral choices--an Elder Scrolls staple--any more than the Empire's overarching existence's does Daggerfall. It's likely that Dominion's principal moral hazard lies in how the Hero goes about unifying the resistance, and be informed by various groups having different degrees of moral rectitude and likelihood of carrying out their resistance.
5. Freedom of movement--to a significant degree--will be curtailed, especially in the early game. (Full freedom of movement occurs in the late game.) No matter how you go about it, the Hero will quickly be branded an enemy of the Thalmor state, and so will need to get quite good at avoiding Justiciars, at least until you're powerful enough to take them head-on. Much of the early game will thus take on elements of getting from Place A to Place B without attracting authorities' interest, and creating and inculcating friendly bases throughout Summerset. The Dragonborn you ain't.
Coll. Incidentally, the Shouts mechanic will be implemented here, but in a vastly different way. It's likely that, in the lore, under the Last Dragonborn's influence, Shouting was developed from a "special skill" to a school of magic built on the Greybeards' teachings.
Coll 2. It's likely that Summerset's traditional joinable factions will be closed to non-Altmer (particularly if they keep this Nazi thing going). I'd bet that the "real" joinable factions will involve some combination of hidden factions and under-the-table dealings with traditional ones closely associated with the Thalmor, and relative freedom for the ones that aren't. So, for example, the Assassins' and Thieves' Guilds will be significantly easier to join than the Fighters' Guild or local Mages' Guild variant (unless that turns out to be the Psijic Order, which has zero known relations with the Thalmor overlords).
I don't really know what's on Bethesda's storyboards, but these ideas seem like they'd make the best possible Elder Scrolls game to me. Certainly, gameplay mechanics in Dominion will have a very different, unique, and interesting twist to them, particularly since what we know of the Thalmor implies that the next game will have to encourage stealth-based gameplay to a far greater extent than previous installments ever did.