Here is what is currently known about the game.
What is known about the development team currently, is that it has a few Valve programmers, a game designer from Double Fine's Massive Chalice and some talented writers. But the most important person to keep in mind: the lead designer Richard Garfield aka the creator of Magic The Gathering.
As for the development itself, not much as known. Apparently, the game has been in development for at least 2 years. It has a sort of closed beta, specifically, there is a build of the game that only a select few(Dota 2 celebs mostly). The game will run on Source 2, making it the first official one released with Source 2 and not converted partially. The game will be released sometime this year(2018), with a speculation that it might be around either at or after “The International” (Dota 2 biggest tournament). It will be on PC, Mac, Linux and mobile devices.
The game will be not free to play. It will have an upfront price (possibly around 20 $) and that gets you the all the starting cards (248), and you will have the ability to buy card packs. All signs point towards the game being more of TCG rather than the CCG which most digital card games are. A TCG or Trading Card Game has a strong focus on trading the cards, which means that cards will have actual rarity rather than just an artificial one. Cards can be purchased through the Steam Marketplace.
Also, the game will have a Compendium, similar to the one Dota 2 has. And even before it's officially released, there is a planned tournament for 2019 with a million dollar prize pool.
In short, it's a conversion of how Dota 2 plays.
The game takes place on three boards, called lanes. You will have the ability to zoom into each lane individually or view all 3 at once.
On each lane, there is a tower and mana pool. It starts at 3 and gets bigger by 1 every turn. A turn is made out of a “round”, a “shopping phase”, and “hero deployment”. During the “round” phase, you and your opponent go lane by lane, playing spells and seeing how combat plays out. After combat in all three lanes, you move to the shopping phase to purchase items (you get gold by killing creep and hero cards). This items will be equipped to your hero card and will stay on them until the rest of the game.
After the first turn, there is a 4th phase, known as “Hero Deployment”. During it, 2 creeps are randomly placed on the board. Each player can deploy available heroes among the 3 lanes. However, the targets for the combat phases are randomly assigned. And apparently, there is no limit how many cards you can have in play.
To win a game, one must destroy two of three enemy towers (40 HP a tower) or kill one enemy tower and then their ancient (which has 80 HP).
Decks are made up of five hero cards, abilities, and summons. Must be at least 40 cards (and there is no maximum limit).
You may have up to three copies of the same card. Each hero adds 3 copies of a signature card. Signature cards are a part of the 40 cards, leading to 15 mandatory cards, depending on the heroes.
As for the cards themselves, they are divided into these type:
Hero, Item, Improvement, Summoning, and Spell Cards.
Improvement cards allow you to summon structure to help a lane (like a barracks to spawn minions or a ballista to damage a tower directly).
Cards are divided into 4 colors:
Blue-Mage heroes and late game powerful spells.
Green-Support heroes and summoning spells, for board flooding.
Red- Heavy attack heroes.
Black-Assassin heroes and cross-board abilities and spells.
The last thing this card game will do differently than other games is that there will be a focus on lore. It takes place in the Dota 2 universe. Lore will unravel as you play the game, heroes will interact with one another. And future expansions will see some of the same cards, but with different art(and probably abilities) representing the change in the story. Again, similar to the way MTG handles its story.
All and all, as much as I like free things, and how my opinion on Valve is soured, I can't say that I don't like how the game is shaping up.
It's different, there are constant promises that “pay to win” will not be a problem and is by Valve. I can say a lot of bad things about Valve, but I can't say that the games they publish are bad.
Also, it would be fun to play a Dota game, which does not require a spare hour for a match and a partially toxic player base.