Critical Saves

Another comment that was made a while ago, this time by Andreas, was that a natural 20 on a saving throw, instead of causing the character to automatically save, should instead allow the character to take no damage from the effect.

This is certainly a decent idea, and doesn't damage the game over-much, except for one detail: there exists the case where a character can save only on a 20. In such an instance, the character either takes full damage from the attack, or takes no damage. This is similar to the case found in many 2nd-edition house rules on critical hits, where a natural 20 did double damage.

That being the case, I offer the following house rule as an alternative that does basically what Andreas suggested, fits the existing rules, and solves the problem just outlined:

When a character must make a saving throw against a spell or other effect, where a successful save causes the character to take reduced damage or a reduced effect (so, also includes the like of "Will partial" saves), a character has the possibility of rolling a "critical save".

If the die roll on the saving throw is a natural 20, this roll is considered a lifeline. The player should immediately roll a second saving throw against the same DC, using all the same modifiers. If this second roll is also a successful save against the effect, the character has rolled a critical save. In this instance, the character is completely immune to the effect, taking no damage or being otherwise affected.