The other idea is more basic (and perhaps more complex to DM.)

Make a number of power words, and depending upon how they are combined the spell has an effect. The words alone do not have exact meanings, they just convey basic ideas of what the spells may be.

Example: Five Words: Fire, Tree, Man, Wolf, Water:

Water+Fire = Obscuring Mist
Tree+Man = Barkskin
Wolf+Man = Wolf form
Water + Man = Water Breathing
Man + Fire + Water = Healing
Fire = burning hands
Water = create water
Tree = Bolster Plant creature.

This would make for difficult DMing, but potentially fun experimentation by players. I would imagine that all the players should be playing casters in this world though, I'd hate to be Steve the Fighter campaigning with 3 wizards who know a smattering of spells and are experimenting with them for the first time in combat!