Very impressive. Apologies to the real person I assumed was a bot so I rudely alt tabbed for a minute.
I never got into hearthstone so I'm not the best judge, but it's very impressive how accurately you replicated the mechanics (from my limited knowledge). I assume that this is a practice project for an original cardgame, right? So much of the card game experience is the bells and whistles to trick the monkey brain into activating neurons. If anything, it might even be more important than the quality of the gameplay strictly in terms of making it.
I know that's really obvious and lots of things could still be placeholders, but that's the main thing that comes to mind. Also I don't think pixel art has enough visual clarity for card games--not unless it's really high resolution.