Inspirations: Psionics, and Seerow.
Alright, this is my revamped system for spell resistance, using psionics/ spell points as a base.
Changes in Spell Resistance
Spell Resistance, rather than being a binary yes/no opposing check, instead reduces the number of power points carried in a spell. When a creature with spell resistance is affected by a spell, the number of spell points invested in the spell is reduced by an amount equal to the creatures spell resistance, with corresponding consequences to the power of the spell. The spellcaster may make an opposed will check against a creature with spell resistance, reducing it's effective spell resistance by 1 for every one higher his will save is than its own. However, if the creatures opposed will check is higher than the wizards, its effective spell resistance is instead increased by 1 for every one by which it beats the wizard.
All spell points lost as a result of spell resistance are gone for the day as if they were cast in a spell, but have no further effect. If the number of spell points lost from a spell puts it below the lowest number necessary for it to be cast, the spell instead counts as the next lowest spell in it's chain.
For example: The 7th level wizard Australwitz casts a lightning bolt at a Drow, which he invests 6 spell points in. The Drow has Spell Resistance 3, which would reduce the power of the spell to near impotence, so Australwitz decides to risk an opposed will check. The wizard achieves a solid 21, but the drow gets lucky and manages a 20. Australwitz has still overborne the drow, however, and its effective spell resistance is reduced to 2, which then negates two of the spell points invested in his lightning bolt. The lightning bolt now contains only 4 spell points, and deals damage appropriately.
So, too complicated, or no?