To follow up with Drake,
The card idea of the month sort of thing does lend itself to getting new cards in, but not in the way you think.
Usually each year or so, a major update is released we see new cards, often they are taken from the card ideas section, sometimes modified a bit or readjusted. We have a major revamp coming called 1.4, it is very likely we will see new cards when it is released. The number of new cards varies between 3 and 12 (6 and 24 if you count the updated form). Also usually some AI adjustments occur too to make the AI better against whatever combinations have been figured out. The bigger the release the more we are likely to see though. 1.4 is a massive change, and as a result, a lot of us are very hopeful, that a massive increase in the cards occur too.