Developer Blizzard is only a week away from its yearly fan convention, BlizzCon. Big announcements related to Blizzard projects are the norm at this event, and it looks like something is in store for the company's digital card game Hearthstone.

Yesterday Polygon received the postcard pictured above in the mail. You can see the full card below:

The card depicts blueprints for a Harvest Golem, a World of Warcraft enemy that has been one of the most popular of Hearthstone's original sets of cards. In the current game, a Harvest Golem costs a mere three mana but also summons another creature when it dies, making it a high-value, commonly-used card.

Alongside the postcard was this note mentioning BlizzCon:

In case you can't read that, the note says:

Making plans for BlizzCon — I hope you like my invention!

Polygon wasn't the only site to receive this cryptic note, though others got different postcards. Mobile gaming site Touch Arcade received a card featuring a demolisher, while Blizzard fan site BlizzPro's mailing highlighted the Repair Bot.

Curiously, all of the postcards we've seen so far depict creatures that are already cards in Hearthstone, so they don't seem to be dropping hints at new cards. Does this mean the Hearthstone announcement at BlizzCon will be focused on changes to existing cards? Or does the robotic thread tying the cards together suggest a goblin or gnome-themed expansion focused on wacky mechanical inventions? Perhaps something a little lighter to balance the deathly focus of the game's first add-on, Curse of Naxxramas.

In September, Hearthstone production director Jason Chayes told Polygon that the game's next add-on would be "our first expansion" with "more than 100" new cards. BlizzCon seems like an obvious place to announce such a thing, but we'll have to wait and see if it happens or how that announcement is tied to this mysterious package.

BlizzCon 2014 kicks off this coming Friday, Nov. 7 and runs through Saturday, Nov. 8.