The problem with saying that "The next game will be about fighting the Thalmor" is that it completely goes against how all the TES games have worked before.
In this game, we have a civil war between Stormcloaks and Imperials, but that's not really what the game's about, it's about dragons returning and a prophecy and killing a god.
In Oblivion, there weren't really any humanoid conflicts going on, except for the cult of one of the gods, it was all about a prophecy and a handful of artifacts and a cosmic battle between different gods.
In Morrowind, it was about a prophecy and a handful of artifacts and fights between gods.
Admittedly, there was a lot more politics and affairs of men in Daggerfall (and Arena), but it was still involving reality-warping artifacts and breaking gods.
At most, I think the Thalmor / Imperial conflict will be a sideshow in the next game, as well, while some new prophecy that can be pulled out of any orifice on Tamriel involving the next batch of world-bending artifacts will pop up to be the main plot. There's no reason it can't be the Hist or the Adamantine Tower or Ceporah Tower or something completely created wholecloth.
I completely disagree with the characterization of "choices" and erasing them in this thread. The Warp in the West had to account for multiple endings...never a good idea, especially in an ongoing series. But since then? Morrowind didn't really have alternate endings. The only choice of any consequence is whether you killed Vivec, a choice which no special plot element has been invented to get rid of. It's covered by characters just saying "Vivec died, or left" when they have to talk about it. The end. The destruction of Morrowind is more "nice job breaking it, hero": it doesn't erase the main quest, it happens because of it. Your actions did have an effect. The Tribunal are gone. Same goes for Oblivion. There weren't alternate endings. The 200 years that have passed weren't to make it less important whether or not you found a way to save Martin. You just didn't. And it certainly had an effect. The empire fell apart, and the current one is a much weaker, smaller empire recaptured one place at a time by the Medes. Parts of it were recaptured by the Thalmor instead. So we don't have three games worth of player-erasing catastrophes to draw patterns from at all.
I don't know if they will do a warp sort of thing for who owns Skyrim now or not. Maybe they will write that both potential rulers died at nearly the same time, after much fighting between the legion and rebellion, and so the new high jarl was some other guy who took whatever side. It would be that simple, really. But the Thalmor are an important part of the plot, not some plot eraser. The main quest is actually the Dragonborn stuff anyway. The civil war is somewhere in between a main quest, and which great house you happened to be in.
You miss the point I was making about the choices - you have more choices than whether you complete the main quest or not.
In Morrowind, you could help or harm the various great houses, and so they needed to have a way to say that no house wound up on top lest that conflict with the player's choices in-game. In fact, you could potentially choose to just go on a killing spree and wipe out whole houses or cities.
You can assume that the main quest is completed, but what if I didn't want to join the Fighter's Guild? For that matter, what if I joined the Thieves' Guild? I can't join both those guilds in the same play-through, so you can't just say the results of those questlines are both what canonically happen (not without another Dragon Break).
The only way to wrap up all the plotlines without Dragon Breaking after every single game is to have a canon intervening event that completely wipes out all possible consequences of the player's free will. That means
rocks fall, everyone dies, even, in the case of Morrowind, literally. The state of Cyrodiil, and what portions of the plot you accomplished, or what guilds you did or did not join are made irrelevant by burning the whole freakin' province to the ground. Plus 200 years of history to wipe out anything but the vampires and maybe a few altmer, dunmer, and bosmer who would nevertheless be quite old.
What if the player kills Paarthurnax? It's part of a potential questline you can take, so it's just as canon as
not killing him. Clearly, Paarthurnax can't appear again without contradicting canon, the way that Vivec had to disappear. Paarthumax says that he'll try to teach other dragons to be peaceful... but he obviously can't do that if he's dead, so obviously the choice you make can have no future impact upon game lore.
So yes, I'm saying Thalmor invades, everyone dies is the way they'll probably clean up the mess potentially made by player choices in Skyrim. Granted, maybe I'm wrong, maybe it's some other cosmic anvil falling from on high, but basically, the only way to tie up the loose ends is to kill off anyone who might potentially have been impacted in more than one way by the player.