The Empire was already a united Tamriel and the Aldmeri
1. Defeated the Empire
2. Controlled them
Literally the Thalmor will actually leave Skyrim if the Stormcloaks win the civil war lol. They could only maintain a hold over Skyrim via Imperial influence. Hence why the Thalmor outwardly support the Empire winning the war.
Also Hammerfell is independent and the Thalmor can’t do shit to Hammerfell lol. Hammerfell and and Independent Skyrim would ally in a mutual desire to not be controlled by the Thalmor.
Also, the Dragonborn is so powerful that whatever the Dragonborn wants to happen will happen. If the Dragonborn tells the Thalmor to bend over and get fucked, they will ask how hard or die lol.
There's documents in the Thalmor Embassy that say that the civil war in Skyrim benefits them.
That alone convinced me to side to with the Empire, because I'm not about to help them divide and conquer and let them have what they want.
The only reason they havent destroyed the Empire in the first war is because they couldnt, and Titus Mede's dumb ass was too hasty with signing the White Gold Concordat.
If the Empire loses Skyrim after losing Hammerfell, it will only weaken further, and at that point the Thalmor will be able to just swoop in and take Cyrodiil and commit genocide against humans there to restore their twisted version of Ayleid reign, then they will turn their gaze towards Skyrim and Hammerfell who are both alone now dealing with rebuilding + internal issues and they will stomp them mercilessly.
Even better for the Thalmor is if some provinces start fighting each other over ancient border disputes that were frozen since Reman's Empire.
The Thalmor would win their divide and conquer agenda and every province is getting smashed then absorbed, one by one.
Dragonborn is out of commission after becoming a servant to Herma Mora like Miraak.