Very good game, but I have some suggestions to make it better:
Effects: Mid/late game enemies could have some special stats that can make them more powerful and replace the exponential rise of health and attack. You could give them certain abilities, such as poison, which could temporarily remove one max hp for just that battle, forcing you to attack. You could also use procedurally generated stats that could scale the further you got.
Bosses: Perhaps every five or ten rounds, there could be a boss enemy. The boss enemy could have powerful abilities, and after you defeat it, you could get an ability yourself or trade it for coins.
More card stats: You could add more card types. Here are some examples. (The cost shows how expensive it could be to use this card or how much you could use it.
HAND SIZE- increases/decreases your hand size. Very expensive.
FREE REROLLS- allows you to reroll in the shop. Extremely expensive.
STARTING GOLD- increases/decreases your starting gold. Average.
SHIELD- gives you shield which could protect you before you use up hp. Cheap.
Gradual Strength: Every time you defeat a boss or something like that, you could get points or another currency to by starter abilities.
Please consider my ideas. I love this game, but it still needs some improvement.