Sounds like a new Argent Tournament themed adventure is coming (or at least being announced) in a couple weeks:
http://massivelyop.com/2015/07/08/he...-in-two-weeks/
I'm guessing it'll be similar to the Naxx adventure with ~30 new cards.
Pretty crazy how fast they're pushing out content for this game.
Edit: Actually, I guess this could be a full expansion too. Didn't realize BRM was just released a couple months ago and GvG was released back in Dec. Hard to imagine another big influx of cards right now though. My money is still on a small adventure like Naxx or BRM.