I wonder what are their plans for the game. Hearthstone began as mostly a "fan service" tool, and turned out to become an extremely popular game (even if atm it's really bad).
I'd be curious to know if CDPR sees Gwent as a fan service game, or if they have plans to make it a competitive game with tournaments etc. The only current issue with the game is the small amount of available cards, but it's a closed beta and it has to be expected. If they'll decide to support it long-term with regular content, updates and tournaments it could have a very bright future.
Oh, I forgot to mention Eternal: Card Game. It's a really good card game that borrow a lot from magic (and a bit from Hearthstone) and can easily be played completely f2p. It's one of the few games where it's possible to make gold by playing against the ai in both constructed and draft, and while rewards are not as good as the equvalent pvp modes it's great to make some gold while playing stress-free, without having to worry about lack of time, possible disconnections, meta-decks etc.