In the release to PC gamer Blizz said that they "have much better ways of communicating now". I wouln't be surprised if the next brawl previewed some upcoming content. Wouln't it be cool if it let you play as monk vs deathknight?
They released the original news via scroll. "better way of communicating." means a modern way, an announcement sent electronically.
They said that they aren't considering making new classes. The argent Tournament isn't where either classes came from, and a new class would be something you'd save for Blizcon.
Overhyping yourself for something 'HUGE" is exactly how you make new content sound worthless . "Aww man, it's new cards? I was hoping for RL flying robots!"
It's going to be an adventure. MAYBE Krip is right that it'll work differently from the other adventures. But it'll still be of the same scale.
Trust how they did GvG. If there's anything more it'll be VERY obvious that something is afoot.