I've always tied critical effects to, well, critical hits.
I have a system of "colorful critical hits" that I'll copy & paste for you. I'm not sure if this suits your taste or not, but my group loves it. Note that this also opens up things like magic weapons that give a bonus to critical severity, armor that reduces it, etc.
This is the last version of the system for 3.x; I have a subsequent version for 4e, but this is notated for 3e mechanics.
Colorful Critical hits in Cydra
Whenever a creature suffers a critical hit in Cydra, check the severity on the following table. This adds both color and deadliness to critical hits and helps scar characters up, leaving them with interesting stories to tell.
Some conditions below are marked with one, two or three asterisks. These designate the level of healing magic required to alleviate the condition, as follows:
*A cure light wounds will alleviate this condition. (Exception: it will not regrow a lost ear.)
**A cure moderate wounds will alleviate this condition.
***A cure serious wounds will alleviate this condition.
When a character in Cydra suffers a critical hit, the severity check is based on how much damage the character took relative to his current hit points. (This is an important part of the colorful critical hit system- otherwise, characters with high hit point totals will never take crits above 1d12 severity.)
Major Wound (less than half current hp): Roll1d12.
Grievous Wound (over ½ hp, still above 0): Roll 2d12
Incapacitating Wound (0 or less, still alive): Roll 4d8
Lethal Wound (dead): Roll 3d6+22
1* --- Break Finger, Toe or Nose; -1 to checks with hand/foot
2*** --- Lose 1d6 Teeth, -2 to Diplomacy, spell failure 2%/tooth
3* --- Break Ribs (1d6); take 1d6 nonlethal damage whenever you run/charge
4* --- Break Fingers (1d5) or Toes (1d5) or Lose an Ear; -2 to checks with hand/foot or to Listen checks.
5 --- Lose Shield (or off-hand weapon)
6 --- Disarmed
7** --- Break Hand, Foot, or Face; -2 to anything using injured part, take 1d4 nonlethal damage when attacking with it
8** --- Break Jaw; -2 to Bluff or Diplomacy, 20% verbal spell failure
9 --- Knocked Back 5’
10 --- Damage to Armor, Shield or Weapon as well as target
11 --- Staggered 1 round
12*** --- Break Lower Arm, Lower Leg or Collarbone; -4 to anything using injured part, aggravating it deals 1d6 nonlethal damage
13 --- Knocked Prone
14 --- Dazed 1 round
15 --- Muscle Damage; take 1d4 Str damage
16*** --- Break Upper Arm, Thigh or Sternum; -4 to anything using the injured part, aggravating it deals 1d6 lethal damage
17*** --- Hamstrung, -10’ speed
18 --- Lose an Eye
19 --- Lose Fingers (1d5) or Toes (1d5) or Nose
20 --- Deafened
21 --- Lose Hand or Foot
22 --- Internal Damage; take 1d4 Con damage
23 --- Stunned 1d4 Rounds
24 --- Lose Lower Arm or Lower Leg
25 --- Lower Jaw Ripped Free; -8 to Diplomacy checks, 75% spell failure
26 --- Gutted; take 1d6 Str and Dex damage, 50% chance to tangle feet each square of movement, which inflicts 1d6 hp of damage to the victim
27 --- Vocal Cords Destroyed; no speech or verbal spellcasting
28 --- Brain Damage, 1d4 points each of Int, Wis and Cha drain
29*** --- Knocked Unconscious 5d20 Minutes
30 --- Shattered Knee or Elbow; completely unusable, requires a regenerate to fix
31 --- Both Eyes Put Out, Blinded
32 --- Spine Shattered, Paralyzed (roll 1d6: 1– total; 2- neck down; 3- waist down; 4- left side; 5- right side; 6- arms only)
33 --- Throat Slit
34 --- Groin Destroyed
35 --- Heart Destroyed
36 --- Brain Destroyed
37 --- Decapitated
38 --- Unseemed from Nave to Crotch
39 --- Head Pulped
40 --- Cut in Half