I am actually thinking about playing a magic user (Big surprise) and basically to keep this magic user balanced, her partial brand of magic is semi-sentient and enforces three rules on her use of magic:

1. There must be some respect for the four elements: Fire, Water, Earth, and Wind. Namely, she can't immediately encase someone in matter or turn the floor in air and have them fall to their deaths. She can do those things, but it takes time. If she is just making like a pen, it is immediate, but if she makes a block of stone it needs to take time or a giant pit it will take some time. Also, in order to create solids, it must touch a solid (And can't be on a person due to a later rule) and in order to create a gas/vacuum space it must touch gas/vacuum space. Also, this also means she can't just set water on fire, but that is kind of obvious. 

2. Magic can not directly effect an unwilling sentient being with notable intelligence. This consent can be implied, or previously stated and she can even make contracts but this is just to stop her from just polymorphing everyone and being completely busted. How this consent is obtained is not relevant, and can be got through threats, blackmail, lying etc. This rule also does not apply to offense spells and healing spells, for what I hope is obvious reasons. - As an addition to this rule, any contract made with another magic users must also be obeyed, though the rules can be bended, and a contract can break the first rule nor any limitation of magic. 

3. Magic can not be lethal (Unless absolutely necessary to save the caster's life or limb) and is almost always not as bodily damaging as its non-magical counterparts. So if you set someone on fire with a magical fire, it is going to hurt and maybe give them some burns, but it going to cover their body in three-degree burns, and if she makes a magical sword and slices some down there waist line, it is going to hurt and probably leave a mark or two, but it going to chop them in half. This applies to conjured weapons. 

And as a note, though these aren't really rules, her teleports either take a few moments of concreted effort towards it and can't be down while she is being attacked, though she can still blink, but blink is pretty much a few feet in a particularly direction. And her leviation is basically limited to anything a human can pick up in two hands with all their strength though she can do it effortlessly despite its weight, so she can pick up a chest, a pen, but can't just pick up a boulder and throw it at someone.