I want to make a suggestion of incorporating probability in the game. Making cards that have, say 30% chance, to cause an effect. This will make things a little bit more interesting as the player has to depend on chance.
There are so many effects in the game including and are not limited to:
Poisoned, Burning, Disabled, Cannot defend, Draw a card, Heal damage etc.

I think making cards that do not have a 100% chance of triggering these abilities might be good. Ofcourse as the card gets costly, either in Casting Cost or in terms of Gold, the chance of triggering the ability increases. Cheap weapons, especially, may be benefitted with this effect. Imagine an unused card like Wooden Spear having an ability like this "20% chance that your hero will take 0 damage when defending" or maybe "20% chance that opposing ally will take 1 damage while your hero attacks" will do to the card.