Stop suggesting DLC of destroying the Thalmor please. It needs to be two sided. If people play a high elf or even a wood elf or khajiit then they may support the Thalmor. I personally hate them but you need to see it from both sides. If a DLC were made for a Thalmor war the developers would need to allow you to fight on either side of it. Such is why I believe the Second Great War should wait for TES VI
They should be joinable but the problem with the Thalmor is they want to undo the world. So a better solution would be the ability to join the Aldmeri Dominion and destroy the Thalmor from the inside and lead a new sect of the Aldmeri Dominion against the Empire.