In past editions, critical hits...
BECMI didn't have critical hits. 1st Edition didn't have critical hits, and included dire warnings that including them would break your game. 2nd Edition (core) didn't have critical hits, although the DMG discussed a possible house rule of allowing a natural 20 to give the character an immediate extra attack against the same target.
It wasn't until "Player's Option" that critical hits became a formal part of the game.
critical hits usually resulted from rolling a natural 20 on the die. I don't really like this, as it makes critical hits happen irrespective of a character's skill. Shouldn't a master swordsman "crit" more than a novice? Shouldn't it be easier to "crit" a less-defended target than one that is harder to hit in the first place?
Well, yes. That's why 3e had the "critical threat" for a natural 20, followed by the confirmation roll. This (plus keen weapons, the Improved Critical feat, and the like) models what you want exactly, since the master swordsman will naturally pass those confirmation rolls far more often than the novice.
Unfortunately, people didn't like it. The confirmation roll was officially labelled
not fun and exiled to the outer darkness. (Yes, that's hyperbole.)
I think I have the solution, and it's pretty simple and straightforward. You score a critical hit if your attack result exceeds your target's AC by 10 or more. You not only hit them, you got an extraordinary success on your attack roll, resulting in a critical hit. Luck is still a factor, but skill is also.
What do you think?
If PCs are regularly fighting opponents with ACs that they can hit with 10 points to spare (or the reverse), then something is off with the campaign - it's either way too easy or way too hard. This approach will make crits really common against mooks and non-existent against real challenges, which isn't good.
Besides, rolling a 20 is just cool.
Honestly, I think either the 3e or 4e models should be adopted. My (slight) preference would be the 3e model, as this gives scope for some additional flexibility (what if, for example, instead of doing double damage you could use your confirmation roll as a disarm attempt?). However, the 4e method is certainly simpler, and does seem to be more fun in actual play.