Here, a more in depth look at the idea, and proof to back up the idea of this as a DLC.
Second Great WarThrought the course of the game Skyrim is a battlefield for the Civil War. The Imperials and the Stormcloaks. Eventually one falls... but not much more happens. No great rewards, no great speech praising you. But there is many a times mentioned war with the Thalmor. Ulfric says that after the Civil War he was going to march his army and show the Aldemeri Dominion what true Mettle is. And at the end of the Imperial quest line General Tulius mentions that he thinks the soilders are going to get a very short break, because the Thalmor don't seem very happy with the Civil War.
Now, how could this play out in lore, how could the Stormcloaks, or even the Imperials wining the Civil War be wrote about in history books without taking a side? The Second Great War... thats how. Let me explane. Say the Stormcloaks won the Civil War, they go and challenge the Aldermeri Dominon, but do not have enough men, or resources to fight them off. But then a treaty is made with the Empire, allowing Skyrim to remain its own country, but only under a protecterate of the Empire. They both agree, and ally against the Aldemeri Dominon. The first Great War was a failure for the Empire, for two reasons. It took place on Tameriel Soil... and they did not have you... the Dragon Born. (Remember if you complete the Civil War questline, you have to be Dragonborn, due to it being rooted in the main questline)
So now as Dragonborn you are appointed as one of the leaders in the fighting. The Empire & the Stormcloaks send there troops to Summerset Isles. The first quest is to establish a beach head, maybe by raiding a costal city? The war Progresses, it being fought on the land of the High Elves, and the Aldermi Dominion. Eventually the war ends, the Empire wins, thanks to the hero known only as Dragonborn. A treaty is signed with the High Elves, The Aldermeri Dominion is severly punished, and weakened. The Empire lifts the ban on the worship of Talos. And then Skyrim agrees to re-join the Empire, now that the regulations are gone.