Blizzard's World of Warcraft-themed customizable card game _hearthstone was originally estimated to launch its open beta last month, but it seems it's still not quite ready yet.

It appears that before Blizzard opens the game up to the masses, it wants to ensure it has the balance as "right" as it's going to be, with no cards being significantly overpowered or having the potential to lead to boring, predictable but reliable paths to victory. It's already implemented numerous changes to how various cards work, and the next patch to the game, due this week, will bring yet more.

The biggest changes in the impending update come from nerfs to several mage and warlock cards, most notably the mage's Pyroblast, which now costs 10 mana to cast instead of 8. Blizzard's thinking behind this change, which will prevent mages from casting Pyroblast until a couple of turns later until they could have done previously, is to encourage "Mages to be more interactive with the opponent to achieve victory, rather than delaying the game until they can Pyroblast."

Hunter decks get a bit of a buff with Unleash the Hounds' mana cost being reduced to 2 from its original 4. According to Blizzard, the "old cost was too prohibitive" to make this otherwise useful card a practical inclusion in many Hunter decks.

The Charge spell has also been reworked to cost 3 mana in order to discourage "OTK" (One-Turn Kill) decks that provide the potential to dispatch an opponent in a single turn without requiring any minions to have been played. "We want the game to be about playing minions and fighting for board control rather than just waiting until you can play your big combo and win in one turn with no interaction from your opponent," says Blizzard.

Blizzard's hope for the new balance changes is to "increase the variety of cards you will see at all levels of play" rather than players feeling that certain cards have to be included in a deck for it to be viable. It will doubtless mean that some players with established, reliable strategies and decks will have to rethink the cards they're using and how they use them, but this sort of thing was, in part, what the closed beta was all about -- seeing how players behaved in the wild, and making balance adjustments accordingly.

"This should be our last big set of balance changes," wrote Blizzard's community manager Zeriyah on the game's official forum. "Once we go live, we plan to make very few card changes, and only if they are absolutely necessary."

There's still no date for hearthstone's open beta, but with this being the last major set of balance changes and Blizzard already referring to the "closed beta testing phase [beginning] to draw to a close," it looks like those of you who haven't been able to get your hands on a key won't have that much longer to wait now.

Existing players, check out the full extent of the changes here.