I'm toying with a sort of feat system for 1E/2E that uses a variable crit/fumble system that can be boosted with experience bonuses.

Basically, as you level up you get skills that can be used when you roll a crit, or when your opponent rolls a fumble.

For example, Biff the fighter may have earned the following skills at 12th level:

Critical Hit

a) Power Strike: If Biff rolls a crit, damage is doubled.

b) Shield Rush: Instead of rolling for weapon damage, Biff can attempt a shield rush to knock his opponent down. The opponent rolls a DEX check at -5. Failure means that he is at -4 to his hit and AC until he regains his feet. Biff must have shield to use this move.

c) Cripple: Biff's attack hits the opponent in a spot that hampers his combat effectiveness. Opponent has -2 to hit and AC until healed.

d) Headbutt: Biff may use a headbutt (for 1d3 damage) in lieu of his normal attack. A spellcaster that's been hit with a headbutt may not cast spells or psionic attacks for an amount of rounds equal to the headbutt damage.

Critical Fumble

a) Disarm: If Biff's opponent rolls a fumble, Biff may use this skill to automatically disarm his opponent of one weapon he is holding.

b) Riposte: If Biff's opponent rolls a fumble, Biff gets a free attack for 1d4 pts of damage (plus bonuses).

c) Bind: Biff's opponent loses his next attack. If the opponent is using multiple weapons, Biff's player may choose which weapon is bound.

As Biff levels up he will be able to unlock different skills, say, one skill at every two levels. In play, Biff chooses ONE of those skills to use when he makes a critical, or when his opponent makes a fumble.