The 'Sanity Point' mechanic from CoC.
You have a pool of say Fortitude points. All spells with Fortitude saves or suffer some condition, now do damage to your Fortitude pool: say 1d6/level of the spell, save for half damage. The following possibilities occur on a 'save or die' spell:
1) You save and your fortitude pool is still positive: No effect.
2) You fail your save and your fortitude pool is still positive: You suffer a specified lesser effect. For example, if you would have died you instead of taking full lethal damage, take full non-lethal damage and fall unconscious.
3) You fail your save and your fortitude pool is negative: You die.
A table could be created to define which lesser conditions a condition defaults to. For example:
Death/Unconscious
Petrified/Stunned
Turn To Stone/1d6 Temporary Dex damage
Stunned/Dazed
Dazed/Dazzled
Dazzled/None
Paniced/Frightened
Frightened/Shaken
Ability Damage/Temporary Ability Damage
Energy Drain/Fatigued
Fatigued/Dazzled
Or whatever.
Additionally, you could define conditions for having your Fortitude/Reflex/Will pool being depleted. As an example, a person with a reflex pool depleted might automatically be under a condition similar to fatigued, or a person with a will pool depleted might automatically be under a condition similar to shaken.