If there are going to be some new cards, (or some new balance for some cards) maybe it will help to look at that website first:
http://www.elementsthegame.com/liststats.phpUnupped most unused pillars: Light Pillar, Sapphire Pillar, Stone Pillar
Unupped most unused pends: Entropy Pendulum, Earth Pendulum
upped most unused pillars: Light Pillar, Stone Pillar, Sapphire Pillar
upped most unused pends: Earth Pendulum, Water Pendulum
So earth and water are, as it is now, the most unused elements in the game (how much they are used in rainbows may be excluded) So it may be a good idea to add one or two usefull non-rare cards for these elements which open up new possible strategies (and of course non-rare, otherwise they won´t change much about how used and "loved" an element will be for the mayority of players (newbies and mid-level-players))
I don´t say that earth or water are weak or something, they are just not as atractive as other elements. (Well Ice stall can get pretty strong with some Arctic Squids (well, they are rare) and earth is a strong rush with some good denial (earth quake and basilsk blood)) They simply aren´t as attractive.
Edit: I made a little mistake, the most unloved element is light (when looking at upped and unupped pillars...)