If you think about, fighting the Thalmor would be possible no matter what side you joined in the Civil War, because the Empire hates the Thalmor just as much as the Stormcloaks. The White Gold Concordnant was only a way of saving the Empire, and they abide by thier ruled because the Aldmeri Dominion could destroy the Empire. I do believe that the Great War would have ended a lot differently if the Dragonborn fought with the Empire. Now I think it would be amazing to have a decent sized questline of espionage type activities against the Thalmor (not exactly sneaking about, but actions that would directly damage their power)leading up to a battle that has you riding into battle alongside General Tullius/Ulfic Stormcloak against the Thalmor in a fight to free Tamriel.