Definitely a cool Idea. My suggestions.
Earth: +1 HP for all your creatures.
Air: +1 attack for all your creatures.
Fire: +1 Fire mana/turn
Water: Opponent's Creatures cost 1 more to play.
Light: You gain 2 HP per turn.
Darkness: Your opponents lose 1 hp per turn and you gain 1 hp per turn.
Entropy: The creature with the Least HP (either side) in play can not attack or be targeted. IF two or more creatures are tied for least hp, this ability has no effect.
Gravity: At the end of your turn, gain control of the creature with the most HP unless you already control that creature. If two or more creatures are tied for most hp, this ability has no effect.
Time: +1 hand size
Aether: You always start the game unless you are against another Aether player.
Life: Your creatures gain 1 HP per turn (does not increase max hp).
Death: If a creature your opponent controls came into play this turn, it loses 1 HP at the end of this turn (unless it is burrowed or immaterial).