The tricky bit about making a collectible card game like Hearthstone is that in order to keep players engaged, you have to keep putting out new cards. Blizzard revealed back in August that the next expansion will add somewhere in the neighborhood of 100 new cards to the game, and while it still hasn't been officially announced, it sounds like we could be hearing more very soon.
"One of the things that we spend a lot of time talking about is pacing," Production Director Jason Chayes told Polygon. "Not just the pacing of the game as it is, but pacing of the rollout of new content. We really want to make sure the pacing is great not just for seasoned Hearthstone players but for new players coming into the game too.
Chayes said the development team was excited about the drawn-out launch of the Curse of Naxxramas adventure mode, which expanded the game by 30 new cards over the course of a five-week stretch. That approach "worked out great," he said, and if the upcoming expansion goes as smoothly, "We could imagine a future where we do a combination of [single-player] adventures like Naxxramas and expansions kind of intermixed to have different sorts of experiences going out over the course of a year."
As for when the next expansion will be announced, Chayes said only to expect something "very soon," which makes this a good time to remind everyone that Blizzcon 2014 will take place on November 7 and 8, just over a month away.