One benefit of the d20 system and its endless permutations is that if yo spend enough time looking, you'll almost certainly find that whatever changes you want to make, someone has already done it.
Like with the subject of this post. I don't think Armor as DR is more realistic or anything, I just like it better--and admittedly, playing Baldur's Gate II has shown me how easy it is to turn everything into rocket tag when there's no way to mitigate damage other than "don't let them hit you"--and so I've been looking for a formal implementation for a while. And while looking around online last night, I found out that Game of Thrones d20 does that while also having shields that make one harder to hit and opposed roll combat, with both attack and defense rolls.
Okay, cool. That means I can dump the damage roll and move to static damage, since I also found
this chart that makes any conversions easy. Just plug in the damage done and the attacker size and it outputs a value. It even accounts for critical hits and variations on critical range and multipliers.
One obvious problem is monsters. Unlike Game of Thrones, where nearly every enemy is human, most D&D settings having plenty of monsters and I can't just turn Natural Armor into DR because Natural Armor is usually inflated to make monsters challenging. I don't know if there's a standard modifier I can apply, like 1/2 NA or 2/3rd NA. Fortunately, in E6 Natural Armor never gets too high, but it's something to watch out for.
A second is, assuming I want to use the variable between attack and defense roll to add to damage--and I do--how to account for penalties to the attacker, which are now also penalties to damage? One way is to make some of them into bonuses to the defender which are ignored for calculating damage. Like, Power Attack then gives the defender a bonus to parry/dodge, but if the attacker hits, then damage is calculated using the margin of success without that bonus.
Or maybe that's not worthwhile and it's easier to have tiers, like "For every 5 by which the attacker beats the defender, they add 50% to their base damage." That keeps hit and damage bonuses distinct while making a skilled warrior do more damage overall beyond the obvious note that a skilled warrior will hit more often. Or even easier, for every 5 over, they move up one size category on the chart. Simple and clean.
This also allows armor penetration as a standard weapon ability or feat. Like, it's easy to take something like
Keen Edge and the keen property and make them armor penetration instead.
I'm not sure if I'll get around to testing this since I'm currently on a huge Exalted kick, but it was food for thought. And after months of turning Exalted into a game about 8bit Nintendo sword and sorcery which, amazingly, works, I might be able to make this work too.