I'm pretty sure everybody who is interested in this kind of thing has his/her own houserules, and I know of at least two D20 works that feature something like this already (Unearthed Arcana included), and since it is a quite interesting idea, I'd like to petition for some variant of an Defense Bonus for the Pathfinder RPG. Even if it's just optional, it would help keeping the whole "magical AC bonuses" race in check by providing characters with their own defense, and would make magical armor/shields and Rings of Protection or Amulets of Natural Armor something special again when found, and not something required to survive.
My personal variant (yeah, it HAD to come at some point, we're all houserule freaks after all, right? ;D) is that I grant everybody half his BAB (rounded down) as Defense Bonus to AC, reduced by the Armor Check Penalty of any armor worn (NOT shields, since shields are integrated into your personal defense). The Defense Bonus vanishes under the same circumstances the Dex bonus vanishes (for ease of adjudication).
Thoughts, variants, ideas? :)
Aargh, the site is driving me batty by cross-posting and moving so slowly.
I've done something like this myself, 1/2 BAB to AC. I really like your idea of ACP as a penalty to the bonus you get from your class.
Right, this is like walking through molasses to get somewhere...sweet taste, but impedes movement like hell. ;D
The idea...not sure where I got it, some message board I think...works currently pretty nicely in my Lone Wolf D20 game. I added a houserule that lets them add half the total armor+shield bonus to Reflex saves against effects that could concievably be blocked by stuff between it and your body...acid attacks, dragonfire, etc. Not sure how well that could be expressed in simple rules, but it serves to give heavy armor a little extra to keep it interesting at high levels, where the Defense Bonus would otherwise outrank it easily (for fighters).