Dunmer: the Nerevarine will come back from wherever he is and lead his people to glory.
They most likely lost most of their population since the Red Year, and the mer do not breed so quickly. Also, they're now refugees in Solstheim, surviving on the kindness of the nords there. To note, those nords on Solstheim are not part of Skyrim, even though they do import mead from there.
My choice, a contest between The Aldmeri (altmer) Domain, Cyrodiil (colovians and nibenese), and Skyrim (nords).
Reasons:
Since the last novel, the altmer have made bold moves by land grabbing Valenwood, and are taking pot shots at Cyrodiil. They seem have clear goals to recreate the Aldmeri Domain, by calling the newly held areas as "The Aldmeri Domain." This expansion is likely to be popular with the mer population, but taking over the domains held by humans and beastmen are going to be difficult. Still, if they take over completely, expect an unclimbable caste system, with purebred and perfect altmer at the top, powerful bosmer families in the upper and middle parts, and everyone else on the bottom working as slaves or practically slaves, with macro-scale eugenics.
Skryim has also been making some land grabs too. Since the fall of the empire, the nords have taken territory from High Rock. They are a warrior race, with a few cities of great economical prosperity and some love of arts. An influx of some of the dunmer may help Skyrim even more, but this is only speculation that some of the dunmer even went to Skryim and were not killed on site. However, a problem with the nords and Skyrim is that it really is not much of a unified area, politically, taking on a form akin to the city states of ancient Greece.
Cyrodiil has historically been the empire, so they have history on their side. An emperor has risen, so they are seemingly preventing the hell and anarchy that was the middle of the 2nd era. It has yet to been seen what will become of Cyrodiil, but I have been getting the hint that when Cyrodiil becomes more stabilized, it will try to regain its former glory, and Titus Mede looks to have proven that he is a good strategist and has ambition, as he was able start off as a bandit, become a warlord, and take over White Gold Tower and (mostly) unify Cyrodiil.
Elswyr I do not see as being somewhat possible, as it's pretty much anarchy there since the Mane was assassinated (huge [censored] deal, mind you).
Black Marsh only seemed content to drive out the dunmer, and the imperial presence from before. The only land grab they did was contested Dres-Black marsh territory in southern Morrowind. If the argonians had the ambition, they would have taken all of Morrowind, as they did at least go as far north to crater that was Vivec City.
Valenwood got taken over by the altmer, and is now part of the Aldmeri Domain.
As for Daggerfall and High Rock, if Skyrim's encroachment into High Rock is any indication, both areas are weak, and likely back to many small city states, ala TES II.
To end, the Aldmeri Domain, the empire of Cyrodiil, and the city states of Skyrim are, most plausibly, the big 3 that'll come in the future, as opposed to one group dominating the others. Black Marsh is likely going to stay extremely isolationist and stay put. Daggerfall and High Rock are likely to be contested territory between the three powers and themselves. Elswyr is likely to be integrated into Cyrodiil and used as a defensive position to surround Valenwood, the Aldmeri Domain will invade through Valenwood and the sea to the south, or is left alone as it is too chaotic and not worth the expenditure to take it over.