Now, both outcomes can't be canon, as the warp in the west has already been done.

I'm thinking that neither an imperial nor a stormcloak victory ought to be canon.

I'm thinking that since the threat of the fqalmer invading the surface world has been established by the warmonger unit, and in game documents... The result of the civil war could be rendered moot by their reclaiming skyrim!!!

Wishful thinking, I'm sure. If anything, the next game will take place at or around the same time as Skyrim, so the war will still be ongoing while we take care of whatever needs to be taken care of in Hammerfell, Elsweyr, Valenwood, or wherever. Probably Hammerfell, but hopefully Elsweyr. Anyway, it's likely that any conclusion to the war will be (as you suggested) imposed by an outside source, a third party. Probably, though, that party will be Thalmor. Maybe Akaviri, but not likely.