In many books, and other fantasy sources, a mage's spell overcome the target based on the mages personal strength, not on how well he executes the spell. One way of modeling this is to make the roll to resist a mage's spell a contest of Wills between the mage and the target.
Also, the current system caps a mage's rolls at 16. This is necessary since stats are being used to resist skills (which are easier to raise), but it does bug some people. Under this system (since both rolls are against attribute) no such cap is necessary.
Strong Will continues to provide the normal benefits, but provides no bonus in overcome the resistance of a spell's target. An advantage called Magic Will is created which provides a bonus both for overcoming resistance and for resisting hostile magic (but does not affect other Will rolls) for 6 points/level.
A mage makes a roll against skill with the usual reduced fatigue/casting
time benefits. Then, for resisted spells, he rolls against his IQ+Magic
Will+Magery-2. The target then rolls against Will based on the stat
listed for resisting that spell, Stat+Strong Will, to resist the spell.
(Mages get a -2 because, for example, a mage with Magery 3 and IQ has a base
skill level of 15, but under this system they get a base level to overcome
resistance of 17. Alternately, one could raise the cost of Magery by
3 or 4 points/level).