My top choice would be 'more cards', which interestingly isn't one of the options to vote for. Our card pool is what, like 400 cards? (12 elements x2 for unupped/upped and about 15-16 in each element) Most CCGs have that many when they are brand new. Now, I do realize there's only one person working on the game, and it's free, so we take what we can get, but I'm just surprised that that isn't even an option in the poll.
Other than that, I'd say that having a supported tournament system is high priority, as are more quests, more PvE things, and some sort of incentive to pvp. Most of the time, if I click on pvp2, there aren't even any players available, and most of the ones that are, are brand new players with few or no upped cards.