Well, if it were achievement-based, it would have to be something attainable, but quite out of reach for many unless they are heavily dedicated players, such as myself. As I have pretty much grown tired of gods and have six of every card, I have devoted much of my time to acquiring six of every upgraded card. This would be a sort of "next step" for me personally.
If it were based on just coins, anybody with enough time could grind level 3 and buy another mark. I'm not sure whether I would prefer this over the achievement-based system, but either way I think it could be fun and challenging for all players.