There isn't anything known about this game outside of the fact that it's coming in 2018 and Valve has released a teaser for the game. Given that it's a card game and it's tied to Dota, you can almost say with 100% certainty that the game is going to have micro-transactions out the ass. In fact, I'm expecting that the game will be free-to-play and supported solely by micro-transactions. That's pretty much the only type of game that Valve makes anymore anyway.
Look out for this one in 2018. You might be bored of Hearthstone (and The Elder Scrolls: Legends and Gwent and...) by then, who knows.