Because of Skyrim I have a lowered opinion of TES. Also don't care for lore anymore. But I would get back to TES if this idea were to happen. Posting just for fun.
1. For some greater reason, Skyrim's climate has begun to change and experts believe that the land will become a land far richer for other reasons than just mining. Maybe Skyrim was cursed all this time and the curse was lifted. Or maybe super divine god whatever has blessed it. Any reason to make Skyrim not cold anymore and rich with opportunity and resources.
2. ( Optional ) All nords are either struck dead so that the nord race is ended, or else something has happened that will soon bring about the end of the time of nords. Also anything that can do away with the god Talos, maybe all the gods gang up on him.
3. The reason for the above two are there specifically to make Skyrim a new land that all of the other territories want a stake in. Each neighboring faction has their own reasons to entitlement of possession of as much of Skyrim as they can get. Some legal clause is found that weakens or invalidates the Empire's grip on Skyrim. Khajiit, Argonians, Hammerfellians, Bosmer, Dunmer, etc. are all racing to Skyrim territory to claim it from the dead or rebuked nords while the Empire and Thalmor are locked in disagreement, filibuster conflict.
4. The player can be any race and choose to support their parent nation, the Thalmor or Empire. Or jump into the land grab and face a wild schedule of dog-eat-dog faction fighting and disagreements with all of the other greedy factions. Everyone has their own interpretation of why they should be in Skyrim. There will come a point where unification is advised, or else a new allied nations treaty is reached. And whatever is to be the future of all this mess depends on the player.