One of the more intriguing optional rules found in the d20 SRD is the Bell Curve Rolls. Idea being that if every time you need to roll a 1d20 in the 3.x rules, you roll a 3d6 instead, you’ll end up with a game that’s “grittier” than the normal D&D experience. What they call “gritty,” I’d call realistic, or at least plausible. 3d6 rolls produce a bell curve in which average results are more common, critical failures and successes much rarer, and small number modifiers (like +1, +2, or +3) make a much bigger difference. In short, the Bell Curve Roll creates a game world in which highly skilled creatures really do succeed more often than unskilled ones, and where training actually matters. You know, kind of like the real world.
But this optional rule is premised on the existence of skill ranks and other mechanics not found in most simpler, retro-versions of the game. Nonetheless, it could still be adopted with a little modification.
If I were to port it over, here’s how I’d use it: the core mechanic of combat, saving throws, and ability checks is the roll of 3d6, plus bonus dice determined by your relevant prime requisite ability score modifier. So, for example, if you have a fighter with +2 Strength modifier, and you’re rolling to hit in melee, you roll 5d6 and take the best three. If you’re not a fighter, you don’t get the combat bonus, but you will get a similar one for tasks related to the prime requisite of your class (so magic-users get bonus Intelligence dice, clerics get bonus Wisdom, etc., provided they’re entitled to a modifier based on their prime requisite ability score.)
I haven’t done the math on this, but at a glance, it strikes me as a good way to simulate the effect of training, expertise, and talent in retro systems without needing to port over a cumbersome skill ranks mechanic… assuming one wanted to try the Bell Curve Rolls option, that is.