Titanic said
You should probably classify what types of magic are allowed (Healing, elemental, etc) and whats the limit.
Alright, how about an element system?
Fire is countered by water, which in turn is stopped by earth, which is vulnerable to fire.
Or perhaps a more school-based system, where mages must train in a respective field first, and can't work other types of magic until they've had training.
Or, we could have a Warp-based system, like in Warhammer 40k. Mages need to put themselves through intense psychological training before they can perform even basic spells, as they must reach into a dark, vicious world, where monsters claw at ones mind in an attempt to send them crazy.
Or, a dust-based system. Magic in its purest form is a substance, which is harvested, refined and used for combat, but said substance is somewhat rare, so you don't have a lot of power with it.