I have thought about trying to use something similar to Mystic Empyrean as a system in a MU*. It uses 7 elements as skills - fire, light, lightning, water, earth, shadow, and air in a wheel. Characters are ranked in each element and has a deck of cards consisting of these skills, so if you have a fire of 4, your deck will have four fire cards. When you want to perform an action you draw a card from the deck. If the element drawn matches the element of the action (fighting physically requires fire, for instance), it succeeds better than expected. If the element drawn is one step away, it succeeds normally. If it's two steps away, it succeeds but only if it's not being opposed or difficult. If it's three steps away, it's a failure.
http://www.mysticempyrean.com/The Balance.pdf
I want to use the system in a game where the PCs are spirits and gods. It encourages specialization in certain areas, so a spirits of fire and war is going to have a bunch of fire cards and some light and air cards, while trying to minimize other cards.