In general, I've been switching so-called "critical hits" away from the common "20 on d20" to "5+ on the damage roll, mostly because I wanted special effects to be a little more common, but also to make it mesh with my general rule of rolling 5+ on 1d6 for something to change. Also, I hate the idea of "natural 20 is always a critical"; a so-called critical should be contingent on what a person is doing, not a guaranteed damage boost.
But there's already so-called criticals in the rules for certain monster attacks, like the purple worm's swallow attack. It's the same rule I included in the King Vulture I posted yesterday: target number +4 or a natural 20, if the attack is successful. This is a little more frequent than just a natural 20 for everyone (minimum 5% chance for successful attacks, but better odds for higher-level attackers.) And after the post on called parries, I thought: why not a called critical?
Any attacker with an appropriate weapon or tool that has a special extreme effect (such as severing limbs) can make a "called critical" attack to try to invoke it. If the attack roll is 4 or more points above the target number need to hit, or on a natural 20 for any successful attack, the extreme effect occurs.
Extreme effects need to be distinguished from simpler effects, like grappling, which only require a normal attack roll if they do no damage, or a 5+ on the damage roll, if they do damage in addition to the special effect.
An alternative critical mechanic is: on a successful attack, if the unmodified roll is greater than the target's Dexterity (or other ability score appropriate for the attack,) the called critical occurs. Again, a natural 20 for successful attacks is always a called critical.