OK,
Sadly, I've thought about this. And here you go....
When in d20 you have confirmed a crit do the following:
Roll 3d6. Add 1 for every full 10 points of damage the attack did (you don't multiply the damage as you normally would). Add +3 for a 3x crit weapon and +5 for a 4x weapon. Target gets a DC 25 fort save to which he can add his armor and natural armor bonus. If successful reduce the crit by one stage ("A" goes to no crit). DC 40 reduces it by 2.
Total is
<=10 A crit
11-13 B crit
14-16 C crit
17-19 D crit
20+ E crit
Roll the critical normally. Every 2 points of RM damage is one hit point of damage (bleeders of 1/round go to 1 per 2 rounds).
"Can't parry" becomes "flatfooted" and "can't dodge, fight defensively, use combat expertise or anything else that helps AC"
"At -N" becomes "At -N/10 (round to more negative number)" (and +N becomes +N/10) If the penalty is just "-20" rather than "-20 to attacks" the penalties are to all attack rolls, skill checks, init, and fort/reflex saves and damage rolls.
"Stunned for X rounds" becomes "stunned for X/2 rounds, round up"
You need to figure out what removes the penalties. I'd say minor magics should remove minor penalties, but be worthless against big things. So for example:
An Nth level "cure" spell can cure up to N in (D&D) penalties but has no effect on bigger wounds. Natural healing will cure 1 point of of penalties per week if you make a DC 10 fort save, +1 per 10 you make it by. A healer can replace your fort modifier with their skill check (take the better one, both take the penalty the character is currently at though). Once it gets low enough a cure spell will work.
Once a person gets to full HPs an additional cure spell (CLW or better) will stop a delayed death effect. As will a heal spell no matter what.
Heal solves all problems.
So someone who has a "D" grapple critical on 81-85 would be:
* Knocked prone
* stunned for 1 round
* disarmed (redundant in 3.5 as stunned does that)
* flat footed (but able to act after the stun ends) for 2 rounds.
Example:
Bob the fighter is hit by a longsword for 12 points of damage. The attack threatens and confirms using standard rules. The attacker rolls 3d6 and gets a 12. He adds one because the attack did 12 points of damage, so the total is 13, a "B" crit. Bob has a Fort save of +6, and an armor bonus of +9 in his +1 full-plate. So he rolls a d20, getting a 12 and adds 15, getting a 27, which reduces the crit to an "A". Attacker rolls d100 on the slashing table and gets a 89. Things look grim for Bob. He gets:
+8 hits ==> +4 damage
stunned for 2 rounds ==> Stunned for 1 round
unable to parry for 2 rounds ==> flatfooted and unable to dodge etc. for 2 rounds
bleed 2/round ==> bleed 1 HP/round
At -10 ==> At -1 to attacks, damage, fort and reflex saves and skill checks.
Other issues:
Large and very large creatures use different crit charts normally. I'd ignore this for "large", use the "large" for "Huge" and the "very large" for gargantuan. Big things just shouldn't be that tough in D&D. Further I'd suggest treating "A" crits as "normal", "B" crits as magic, etc. rather than using the actual categories.
