In a recent talk with Polygon, Hearthstone production director Jason Chayes mentioned their plans for the next expansion. This will include 100+ new cards, possibly spread out over as much as a year in a roll-out which could contain both single (Naxxramas-like) and PVP content.
This is due to the fact that the 5 week Curse of Naxxramas launch event proved to be a success, likely setting the release model for every future Hearthstone expansion to follow in its footsteps. Of course, depending on the types of cards and the final number, Jason mentioned that pacing will be important for both hardcore veterans and new crops of players.
Another interesting statement was the idea that further down the line they may entertain the idea of introducing new modes of play (2v2, maybe? Or 5-man dungeons or raids?), which I’m sure Blizzard will pull off. One last idea which was touched upon was the notion that the continuing number of cards might lead to a Magic: The Gathering scenario, where there will be a limit to the type of cards to play in certain modes. That particular problem, though it will exist at some point, is still too distant to worry about.
One final consideration is that with this year’s BlizzCon quickly approaching (Nov. 7th) we may get more details and even card previews.