In trying to come up with a viable idea for making AC actually mean something, I have been kicking around the following idea:
If you wear Light Armor:
AC<20 = 2% of incoming physical damage is ignored
AC 20-29 = 4% of incoming physical damage is ignored
AC 30-39 = 6% of incoming physical damage is ignored
AC 40-49 = 8% of incoming physical damage is ignored
AC 50-59 = 10% of incoming physical damage is ignored
AC 60-69 = 12% of incoming physical damage is ignored
AC 70-79 = 14% of incoming physical damage is ignored
AC 80-89 = 16% of incoming physical damage is ignored
AC 90-99 = 18% of incoming physical damage is ignored
AC 100 and over = 20% of incoming physical damage is ignored
If you wear Medium Armor:
AC<20 = 5% of incoming physical damage is ignored
AC 20-29 = 10% of incoming physical damage is ignored
AC 30-39 = 15% of incoming physical damage is ignored
AC 40-49 = 20% of incoming physical damage is ignored
AC 50-59 = 25% of incoming physical damage is ignored
AC 60-69 = 30% of incoming physical damage is ignored
AC 70-79 = 35% of incoming physical damage is ignored
AC 80-89 = 40% of incoming physical damage is ignored
AC 90-99 = 45% of incoming physical damage is ignored
AC 100 and over = 50% of incoming physical damage is ignored
If you wear Heavy Armor:
AC<20 = 8% of incoming physical damage is ignored
AC 20-29 = 16% of incoming physical damage is ignored
AC 30-39 = 24% of incoming physical damage is ignored
AC 40-49 = 32% of incoming physical damage is ignored
AC 50-59 = 40% of incoming physical damage is ignored
AC 60-69 = 48% of incoming physical damage is ignored
AC 70-79 = 56% of incoming physical damage is ignored
AC 80-89 = 64% of incoming physical damage is ignored
AC 90-99 = 72% of incoming physical damage is ignored
AC 100 and over = 80% of incoming physical damage is ignored
If you wear no armor, there is no incoming damage ignored unless you have evasion or improved evasion. If you have evasion, incoming damage is ignored as though you were wearing light armor. If you have improved evasion, incoming damage is ignored as though you were wearing medium armor.
This is just an idea – the numbers are far from perfect.
Someone recently posted that it seems ridiculous that a fighter can obtain the same benefit from wearing pajamas as he can from wearing plate mail, which I agree is ridiculous. A fighter (for example) wearing plate mail and carrying a tower shield should be able to ignore a large amount of incoming physical damage, certainly more than a fighter carrying two weapons and wearing pajamas.