In an interview with PCGamesN, the game's director Eric Dodds spoke about how the new Dragon cards coming in the next Hearthstone expansion Blackrock Mountain next month were going to shake up the game and how they tailored the expansions to balance as well as expand the game.
He said that their aim with the expansion whether they're smaller like Naxxramas and Blackrock or larger like Goblins vs Gnomes, is always to shake up the metagame and to make players want to create new deck types by asking what the new cards can do. He also said that they aim to introduce cards to solve problems that they've identified in the metagame, often working on cards to shift that balance around three to six months in the future of Hearthstone and inventing cards to cover possible metagame shifts that could come up, although he admitted that they're often taking a guess.
On the subject of nerfing cards Dodds said that unless a card takes the game "into an unhealthy place" that they don't think about nerfing or changing it. He gave the example of the Undertaker card which he says the team felt was being played in too many decks, so it got cut down to be not as ridiculous as it once was. He said that the Dr. Boom card which many saw as being too overpowered as well was not considered at the moment to be a card that made the game less fun to play, so they weren't thinking about touching it. He also said that cards would be changed rather than removed from the game.
Blackrock Mountain is going to be released next month sometime in April and can be pre-ordered now.