Another great war, depending on the side you picked in the vanilla game. And in that, we should be allowed to join the Thalmor if we want. I'm picturing large scale battles, not thousands, just maybe 50 on each side. ACIII can do it. Why can't Bethesda?
Completely different engines make this a huge task to tackle. It's the same thing as asking why there are no long cloaks in the game. The dynamic animations needed for this would take a lot of code re-writing. Also, maybe it's just me (360 user) but my machine barely gets it through some of the larger battles in the game, especially if I'm in a bright, snowy area.
But since you like large battles, http://gamingbolt.com/watch-500-draugr-200-skeletons-500-dwarven-warriors-in-this-great-battle-of-skyrim!
I would like to see a DLC involving the Falmer/Dwemer conflict. More specifically the Falmer (since the Dwemer are extinct and I hate those little buggers).