Since there is a very low amount of cards in Elements compared to most card games, you can't just use an existing card, change the element, and plop it as a new card. It might pass in games like Magic: The Gathering, but in Elements all cards are unique (even Short Bow and Hammer - one goes past Wings no matter what, the other can gets bonus from two instead of one elements). Theme is a good guideline to make a card unique. For example, with Rain of Fire, it can deal massive amounts of collective damage. Fire is known for doing lots of damage, whether to creatures or to the opponent. Earth is a little trickier, but it seems that Earth is a very defensive element. With Rockfall, you can make it deal like 1 damage to each enemy creature, then automatically burrow all your creatures. This is just my example, however, so you should come up with your own thematic, unique idea.
