Probably my favourite MMO-related video (admittedly I don't watch many) is that trailer for Final Fantasy XIV that comprehensively wiped the previous iteration of the game from the map. It was a typically, beautifully overblown way to make up for a bad game launch, and it paved the way for the massive, reputation-restoring overhaul that is A Realm Reborn. The above trailer for FFXIV's first expansion, Heavensward, isn't quite as apocalyptic, but it does set the scene nicely for the seemingly sizeable add-on, even if I have no idea what's going on.
If you've been following the story of A Realm Reborn, you might want to ensure that you've finished it first, as the above video reveals what happens at the end. It's the opening cutscene for Heavensward, see. The expansion, incidentally, adds flying mounts, new classes, and a bunch of other stuff, as I mentioned a while ago here. You can see some of that in the trailer below.
Heavensward is out June 23rd.