Demon Magic
A mage casts spells by summoning a demon to do what he designed the spell to make it do. The focus is not of what it does, but the cost. In exchange for the task, the demon is given freedoms depending on the power, difficulty, type and scale of the spell. This makes every spell a risk. Is it worth it to cast a spell to lift boxes when the demon who does the heavy lifting will go on to smash up a good chunk of your warehouse? There is also the possibility that the spell is simply not enough to contain certain demons who can only come through the most demanding of spells. Summon a mind controlling demon to turn the enemy army on itself? You might end up with it ruling your country since a demon with power on such a scale is both clever and strong enough to escape the bonds of nothing short of the most skilled sorcerer. This makes the user look for alternative solutions to their problems that either don't require magic or allow them to summon low power demons. Instead of an epic level mind control demon you could summon a demon who simply looks like he could wipe the army in one go and make them break ranks through sheer intimidation. Resourcefulness is key.
The last system is the one I'm most interested in, but I think the others have a lot of potential. My big issue is classifying exactly what they do. 
Demon systems do humble the casters because of their reliant s on a outside and unpredictable source for their power. The Al-Qadim Ginnie system used the same type of mechanic. In the book Master of the Five Magics there was a great mechanic involving demon summoning, every time you summoned a demon there is a battle of wills the stronger the demon the harder it is to beat them and each time you summon the same demon that demon has a better chance to beat you the next time he is summoned. This keeps the caster from pulling out the big guns every time and even a week faithful demon will eventually become unreliable. If a battle of wills is lost the demon can do as they please.
Well, point of this system I've put up here is that
every spell sets the demon free within certain limitations such as time, area, or capabilities, depending on the spell. A typical magical duel with this system has three major factors: 1) a mage's ability to counter their opponent, 2) their magical resources, because, even though there's already a cost they still need to construct their magic, and 3) their ability to deal with the consequences of their magic. A duel between competent mages will often result in swarms of small demons throughout the battlefield.
