Critical Saving Throws
Unlike attack rolls, which have critical hits and critical misses, saving throws don't have critical successes or failures. Using this variant rule, you can allow critical successes and failures on saving throws.
Critical Success
When a creature rolls a 20 on their saving throw against an effect, they automatically succeed on the saving throw, even if the result of their save would not meet or exceed the save DC. Additionally, if they would suffer any effects on a successful save, such as half damage, they treat the effect's damage as if it had rolled the minimum.
Critical Failure
When a creature rolls a 1 on their saving throw against an effect, they automatically fail the saving throw, even if the result of their save would meet or exceed the save DC. Additionally, if the effect would deal damage, they treat the effect's damage as if it had rolled the maximum.
Features That Grant Critical Saves
Certain features, such as the Explosives fighting style or the Heavy Weapon Specialization feat, grant the ability for certain saving throws to deal critical damage. If you are using this variant rule, instead have them expand the critical hit range by 1. For instance, while using this variant rule, a character with the Explosives fighting style would have a critical hit range of 2 on their grenades. Consequently, creatures that roll a 1 or a 2 on the saving throw against their grenades would critically fail their saves.
Additionally, you can consider allowing weapons with the keen property to increase the critical hit range of their burst or rapid properties.
Saving Throw Checks
If you use the Saving Throw Checks variant, you would instead allow effects to critically succeed if they would critically hit, and critically fail if they would critically miss, as appropriate.