...and it probably will, what do you think is going to happen to the mannish realms? Most of their fates hinge on the outcome of Skyrim's civil war, and while an event or two will probably render the outcome of the civil war void, I think BGS just might split Skyrim up, as the civil war only worsened the rift between the Old Holds (Stormcloaks) and the Reach (Imperial Loyalists). Ulfric's death would only make him a martyr, and while without his leadership there would be infighting amongst the remaining Stormcloak leaders, and without the significant reinforcements promised to Solitude, the Old Holds are lost to the rebels.

On the other hand, the Reach and Whiterun may be threatened by the Forsworn, they enjoy stability and Imperial authority, and all the benefits that come with it, along with the unwelcome presence of the Thalmor. Even if Whiterun and the Reach fall to the Stormcloaks, they are spread too thin to fight off eventual Imperial reinforcements and those still loyal to the Empire, not to mention the Forsworn. The war truly has not clear victor, Season Unending indeed. Because of this, and the description of Snow-Throat being sundered, kingless and bleeding, I think the Stormcloak Rebellion will lead to a split between east and west.

The situation would be similar to that during the Interregnum, The western holds and most likely Whiterun falling under Western Skyrim, and the eastern holds falling under Eastern Skyrim. This of course could be topped off with any sort of event, from Falmer clawing their way to the surface, to Solitude becoming a cold, haunted place under the influence of the Wolf Queen, to border wars with Hammerfell and High Rock. What effect could be slow break up of the Empire have on Cyrodiil, Hammerfell and High Rock, not to mention the assassination of the Emperor, by a member of the Elder Council no less? The end of an Empire?

I would assume that High Rock would break up into the nine nations that make it up (the eight Breton kingdoms, and Balfiera). If these kingdoms want to have any chance of success they will have to find a way to unify. They could form a High Kingdom type polity like in Skyrim, but I feel that the Breton kingdoms are to independent to get along under that system. I think that the best bet for High Rock would be a confederation, with each nation retaining sovereignty and an independent ruler, but with a unified currency and military, and a congress of sorts with representatives from each kingdom. Balfiera's independence may be a problem, due to its high population of Altmer. I doubt that the people of Balfiera would want to join the Dominion, as they have been separated from Alinor for so long that they likely have no allegiance to Alinor, plus I'm pretty sure the refugees killed during the Night of Green Fire were headed to Balfiera, so its probably filled with anti-Thalmor dissidents. No, what I'm concerened about is that the Bretons might think that Balfiera wants to join the Dominion, which might lead to a great deal of suspicion and distrust, which won't be good for anyone but the Thalmor. I also think that the Dominion might attempt to annex Balfiera regardless of the allegiances of its citizens (because Balfiera would be a great strategic asset for the Dominion), and attempt to explain away any invasion by saying "We only invaded because we needed to unite the Altmer of Balfiera with their ancestral homeland". More than enough real-world nations have done that.

Just a clarification, the loyalist holds aren't the reach, only the one that Markarth is in, and it is unlikely that the Forsworn would attack as far as Whiterun, as it is quite far away from the Reach. But that being said, Markarth is definitely under threat from the Forsworn, and if it gets taken over by the Forsworn, the Western Nords would be in big trouble, as they depend heavily on the mineral wealth of the Reach.