As far as the new cards thing goes, it goes beyond the numbers, as well.
I voted on all 3 sections of PvE content and a PvP ranking system. These things, I think, would make the game more enjoyable for longer.
However, I also voted for better game balance, and my idea of how much of the balance needs to be done is with new cards. For instance, I feel that there is an obvious imbalance with regards to shields. Many elements have no shield that can lower damage, and even more have no choice in what shield they can have. Aether doesn't even have a shield that can stay in play for the rest of the match once played. Light and air seem to have a nice number and variety of shields for the current number of cards.
I would also think every element would need some form of creature and permanent control to be on equal footing (not that it all has to be done in the same way or for the same cost, just that they need something to kill what the enemy has). Darkness does this very well, I think, with steal being either great, or maybe terrible (I hate when I have my weapon out, don't want their's, but its going to hurt me if I don't take it), and liquid shadow able to poison...if you are willing to let the enemy get some extra health. It's solid, but potentially costly control, which fits darkness well. Correctly adding control to the elements that lack it is not something that can be done without adding cards.
What's more, I might argue that the elements that people say they want cards for are the elements that need cards the least. After all, say fire is very popular because of its strong creatures and variety of control options. Many people will play fire, and those people will say more fire cards are exactly what they want. However, in this situation, fire would be the element that needs cards the least, not the most. Fire would already be so strong that many people used it at the exclusion of everything else, and cards for the other elements, to balance them, would be more needed than the fire cards that people ask for.