Magic broken up into orders, with each one getting a specific symbol. Primal is a straight line... Nature a triangle. Elementalism a square. Summoning & Abjuration a pentagon/pentagram. Visions a circle. Evocation/Wizardry a hexagon. Mind magic, oddly, is an octogon. None of the current forms of magic are symbolized with a heptagon. A cross would be an appropriate symbol, too.
Harnmaster magic had 6 areas, which were somewhat broad. Air/Light/Illusion, Fire/Pyrotechnics, Metal/Artifice, Earth/Life, Water/Hydrotechnics, and Spirit/Knowledge/Mind.
The resistance check for a spell is equal to its casting difficulty. Which is equal to the mana cost. What you resist with will vary from art to art.
As a general rule, the effect of the spell is equal to how much the target fails the check by.
Long lasting spells take awhile to cast. In general, take the desired time interval, reduce it one step, and that's how long it'll take to cast the spell. For every step you make it longer lasting, add +5 to the difficulty.
For a really loose magic system, the various schools teach Words or Arts. These are single name things like Summoning, Illusions, Fire, etc.. Each comes with a description of the sorts of things that word is good for. When combining Words, use the one with the lowest rating for your skill check. There will probably be some overlap between schools, like Wizardry and Elementalism both including Fire.