Opening ceremonies will at least have the name of the xpac, an overview of what kind of new zones will be in the game, and any new classes/races, but not in any depth. It'll literally be like, "RAH, OGRES......" <wild cheering> "RAH, MURLOCS..." <wild cheering>..."on to Hearthatone!"
So I assume trailer? Like the MOP one. It reveals like new lvl cap, zones, races, class. Like an overview of features.