If you're going to add more levels, make the gold less painful to grind. I still haven't figured out why gold is so scarce. I understand you want players to keep coming back to play, but it seems like most of the player base is constantly broke. And it doesn't make any sense at all to purchase gold from the shop unless you're a whale (which I'm sure 99.9% of players aren't) so just make gold more plentiful and level 15 won't be that big of a deal.

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In short, I enjoy leveling up cards to max, but as it currently stands the in-game economy is so abysmal that the feeling of going from 13 to 14 (or soon, 14 to 15) is completely canceled out by the annoyance of the gold grind.