I'd like to see a huge, realtime event like the opening of the gates of Ahn'Qiraj, but unfortunately if they don't have the new server code in place by the end of 1.23 it's just not going to happen, due to the 40 character display limit. If they don't have an in-game event, they should definitely make sure to release one hell of a cutscene leading into 2.0.