Quote Originally Posted by Ignimortis View Post
For some reason, AC would make a LOT more sense if, say, it added your proficiency bonus for martial classes (and only them). But it doesn't.
It seems like it would need some pretty intricate wording to work as I think you're asking it to. I'm not against the idea, but I think it could go a couple of different ways, depending on how complicated you want to make it (and how future proof you want it to be).

If the game remains static, and there's no new classes added, it'd be rather simple to state: If you're playing a Barbarian, Fighter, Paladin, or Ranger, you can add your PB to your AC while wearing armor. If you multiclass with any class not listed, you immediately lose that benefit. (I suppose you could create a feat that removes the MC limitation, since at least then it's a decision point.)

Future proofing might be: If your class (not subclass) provides the Extra Attack feature, you can add you PB... the rest remaining the same.

I was originally going to include Rogue, but I think they're in a good position defensively as is. Obviously, Monks don't wear armor so wouldn't apply. Barbarians get a choice - unarmored, they're easier to hit, and thus more likely to be, creating a natural taunt and using their DR to survive. Armored, they're harder to hit, and can more reliably use Reckless Attack, generating a slightly higher miss rate with their better AC.

Of course, you could take away the armor limitation in the ability, but I think Monks would become far too defensive...

Another option I thought of was MCing out of a martial class would divide the PB bonus in half. You still get some benefit, but not as good. We don't want to kill ALL the sorcadins, right? ;)