This is my personal opinion:
- Any DLC regarding the Thalmor should be independant of the Civil War, but it should of course reflect any decisions you've made during the game (Is the Emperor alive, which faction fights the Thalmor, etc). Some people simply don't want to be locked out of content because they don't feel obligated towards choosing a side, which is a reasonable stance given the moral ambiguity of the Civil War.
- If the DLC is about the next Great War, it needs to be done carefully. If a new zone is created, there needs to be a logical reason for the player to visit that zone during war time. It will also likely involve populating the current map with more forts, dungeons, etc. A combination of both will probably suffice
- If it is NOT about the next Great War, there's certainly less limitations. Perhaps you're visiting an island near Summerset? The possibilities are much more varied here.