As Hearthstone: Heroes of Warcraft approaches its two-year anniversary, Blizzard is preparing to implement the biggest changes the popular digital card game has seen thus far. In a conversation with Polygon last week, Blizzard revealed plans to add formats to Hearthstone beginning with the launch of the game's first expansion of 2016 later this year.

The idea of formats will be familiar to people who have played traditional card games like Magic: The Gathering; essentially, they're rules that determine which cards from the complete set can be used in any given game. In the case of Hearthstone, there will be two formats introduced in 2016: wild and standard.

Wild format is the traditional mode that players have used up to now, where every card from all expansions and adventures is allowed. The new standard format will only allow cards from the Basic and Classic sets and any expansions or adventures from the the last two calendar years. Both wild and standard will have separate ranked and casual play modes.

"The volume of feedback that we've been hearing is, 'Hey, it's getting more and more difficult to get my friends to play Hearthstone,'" lead designer Ben Brode told Polygon in an interview last week. "We really want each new thing that we release to really shake up the meta-game. It's one of those things that very slowly, over time, it gets harder and harder to do that."

When standard format is introduced this year, cards from the Curse of Naxxramas and Goblins vs. Gnomes sets will not be allowed in that format. Blackrock Mountain, The Grand Tournament, and League of Explorers cards will be allowed, along with Basic and Classic cards and cards from the as-yet-unnamed new expansion. Blizzard is calling 2016 "The Year of the Kraken" as "a thematic way to refer to the first year of standard format year." As card sets are rotated out of standard with each year's first expansion, Blizzard will give each year a new name.

Once a set has been rotated out of standard, it will no longer be purchasable with either real money or in-game gold. Instead, Blizzard says players will be able to craft any non-standard cards they want to use in wild format using the game's arcane dust crafting system. This includes cards from adventures, which were previously uncraftable.

Adventures themselves will also no longer be purchasable. If you want to experience the single-player content adventures offer, you'll need to buy them before they're rotated out of standard mode. Brode said Blizzard may consider making non-standard adventures purchasable again in the future, but the developer wants to avoid confusing new players with options that aren't as useful to them.

Arena, the Hearthstone mode that allows players to put together a deck from random card choices, will remain in wild format, as will most of the weekly Tavern Brawl updates. However, standard format will be the main focus for Hearthstone tournaments and esports, as well as where Blizzard will push new players for an experience that requires less catching up when you initially start the game.

"Most of Hearthstone isn't changing," Brode said. "You can still play Tavern Brawls with all your cards and solo adventures and challenge your friends and play in wild mode. We're basically just adding a new way to play if you want a meta-game that changes more frequently or don't have all the cards and want to get into a mode where you don't feel like you have to go get every card ever made. That's really what standard is for."

In addition to the fresh formats, Blizzard is taking this opportunity to introduce further changes to Hearthstone. Most notably, players will finally get a long-requested boost to the number of deck slots available. Players will now be able to create and save 18 total deck slots, up from the current nine allowed.

The Hearthstone team will also use the deck rotation each year as an opportunity to review and tweak cards from the Basic and Classic sets, which will remain part of standard format forever.

"We have a list of cards that we're looking at," Brode said. "This will be the biggest balance change that we've ever made to Hearthstone. It's not going to be a massive percentage of the set or anything like that. We have to hit a lot of cards, though. It's definitely going to be significantly less than 20 cards. It's not going to be even close to that number. It'll be more than two. We're still trying to get that final list and still tweaking things."

Brode pointed to classes like the druid as a key example of what the team hopes to change.

"Some classes have too many cards that are just too high power level," he said. "It makes it very hard to make new cards for classes like druid, because they have so many really good cards in Basic and Classic. We have to do some nerfs to make sure we can still see a different druid deck as a new set comes out."

Standard and wild formats will be introduced to Hearthstone when the first set of new cards for 2016 launches. Blizzard isn't yet ready to talk about when or what that will be, but Brode mentioned that the company is targeting a spring release. An FAQ for the changes said the expansion will be "really really cool."

For more on Hearthstone, check out our YouTube playlist of goofy videos on the game.