Thoughts on a magic system.

The seven elements are divided into the four terrestrial (Earth, Fire, Water, and Air) and the three celestial (Light, Darkness, and Aether).

Like all natural living things, people consist of all four of the terrestrial elements, as well as trace amounts of the celestial ones. As such, it is possible to use any of the terrestrial elements to heal. In general, healing spells of a given element will be significantly more effective on people with an alignment toward that element.

(For a tabletop RPG only:) While it is possible to heal directly using the celestial elements, this is generally inefficient due to their scarcity in the human body — although a knowledgeable and skilled user of Aether could, e.g., teleport a poison out from one's veins, or a Darkness-mage lifedrain an illness.

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This could possibly have some interesting effects on game balance. Like D&D4e, everyone has some limited self-healing ability; one can also heal one's comrades if they are coelemental, but this leaves the party as a whole excessively vulnerable to the opposing element, which seems a nice potential balancing factor. Someone who is balanced is equally resistant to all forms of damaging elemental magic, but also to all forms of elemental healing. (See also: 3e warforged.)

The three celestial elements are really just there to have someplace to put the more interesting status-afflictions and utility magics; the ability to use them to mark particularly frightening attacks (a black-flame breath attack which does not char, but causes metal to fatigue, stone to crumble, and flesh to wither?) and plot-significant enemies (I needn't elaborate) is merely a pleasant side-effect.