As the title says, this thread is purely speculative, but I think the next expansion will be themed around the Old Gods, based on two clues.
Many expansions have cards that tease the setting of the next; for instance, Elven Minstrel and his summon line "There once was a man from Gilneas". The teaser here would be more obvious, though: Puzzle Box of Yogg-Saron.
The second clue is much more interesting, though. However, it contains spoilers of Tombs of Terror, so I'll put it... Well, inside a spoiler.
When you're about to defeat the last Plague Lord, Thezan, he plays a card called The Final Plague?!, which summons all four Old Gods.
Maybe these two clues are too obvious to be real... But what do you think? Do you think Madame Lazul is playing the other E.V.I.L. guys and her true intentions are to bring back the Old Gods yet again? Or all of this is just a red herring?
I'll be glad to read your opinion about this theory?