I've always liked these things ... so here are the rules i use in BXDH:
FUMBLE (You rolled a natural 1 to hit)
Roll 1d6:
1-3: you are disarmed/weapon is temporarily unable to be used
4-5: enemy gets a free attack against you
6: your gear is damaged [it gets save, failure is broken, success is damaged until repaired]. If using a ranged weapon 50% you need to make an immediate ammo usage check instead.
CRITIAL (you rolled a natural 20 to hit)
You hit and roll double damage dice
- not double the total damage, just double the dice
- for example, if you do 1d8+2, on a critical you do 2d8+2
I do that to keep it simple
I'm considering this instead - roll 1d6:
1-3: double damage dice
4-5: act again, right now (attack, disengage, chug a potion, whatever)
6: combat maneuvers - you disarm the enemy, break their armor, push them somewhere or get to move yourself. Likely narrated by the GM with Player input? I don't want this to overshadow some of the character's special abilities.
In Sorrow, the levels of success (Crits and Fumbles):
Legendary Failure: the enemy just straight up damages you
Critical Failure: on the sent count, the enemy gets a free attack against you
Critical Success: do maximum damage and enemy count is reset
Legendary Success: do damage directly to VIT and stagger the enemy
I like the variable nature I'm using in BXDH, but the defined actions in Sorrow make for a smoother experience. Sorrow is already more complex, i feel adding complexity isn't going to make it more fun.
In both systems, however, monsters almost always have something more interesting that is monster specific for critical (legendary) hits in combat. Giant worms swallow you, ogres throw you across the room, and so on. Sometimes this is determined by the monster rules, but often it is entirely narrative and based on what makes sense at the time.
I used to use the hackmaster tables and for a while the WHFRP tables ... but they are just too much. simple and a little more, not an entire subsystem.