Cata had literally nothing of the sort, it was just a bunch of big screens.
Legion was revealed at Gamescom, also nothing of the sort.
Only expac reveal so far that remotely had any kind of related artwork built into the main stage was WoD, which had the statues from the Dark Portal on the sides of the stage.
Bear in mind that is NOT actually the main stage that the symbol was shown on, it's the esports/arena stage.