Mark CMG
Creative Mountain Games
Honestly, I think either the 3e or 4e models should be adopted. My (slight) preference would be the 3e model, as this gives scope for some additional flexibility (what if, for example, instead of doing double damage you could use your confirmation roll as a disarm attempt?).
I think I am going to add a new option based on your idea.
My solution? Natural twenty is a crit with no confirmation needed and the player gets the choice to either automatically do maximum damage or roll to do up to double damage. Non-PCs, except for "named" villains and legendary creatures, do not have crits.
Alternately, an attacker who rolls a Critical Hit can choose to force an opposed Strength or Dexterity roll to disarm the opponent, when appropriate.