I'd say none of the above as well. If the Dark Brotherhood can
I'm not sure how a group of five assassins will occupy all of Tamriel.
They were once spread in pretty large numbers in every province.
But to take, occoupy and enforce their rule over all the continent?
They neither couldn't nor would want to do that.
Yus. Didn't know you were talking about an occupying force at first lol. They were once wide spread though.
If the Hist wills the Argonians to conquer Tamriel.... that would be pretty bad! Then again the hist does lose it's influence outside of Black Marsh. That being said Black Marsh is easily the only province impossible to conquer.
So far out of the powers who occupy more than one province the Aldmeri Dominion has the Empire by the balls and with
Nords. They've done it before, they could do it again.
Skyrim does not have to be independent for you not to support the Empire. There are plenty of Nords that live in Imperial holds most notably the Companions and they don't support nor deny the Empire. The fact is Skyrim is much better off under the Empire excluding the current events with the Thalmor and Talos.
Akaviri and Tosh Raka of the Ka'Po'Tun.
The Akaviri would annihilate the Empire and the Dominion.
Scary enough, without Vivec being around, the Akaviri could easily rolfstomp the whole of Tamriel like a Wack-A-Mole game.
Of course. The Ka'Po'Tun have become the dominate Empire in Akavir. Tosh Raka even said he wanted to invade Tamriel, so that might be the plot for TES VI.