Contact a lawyer

Disclaimer: This is my opinion and is not legal advice.

I'm pretty sure the best thing you can do is contact a lawyer.  Even that doesn't make you immune.  I can't imagine Blizzard is going to give you free legal advice.

All I know is I've heard of other games getting sued for being too similar to other games.  So, it's possible to get sued.  You probably just need to make it unique and design it yourself.  If you're clearly just copying Hearthstone then you're probably in trouble.