If you want an idea to work with that has a bit of the Dark Souls feel, I have a suggestion.
If you give up 1 attack, you reduce all incoming damage until the start of your next turn equal to the shield's AC bonus + your BAB. For instance, if you have a +2 heavy shield (4 AC) and BAB +12, you could make a full attack and take your first 2 attacks as normal (not getting any damage reduction from your shield if you trigger a readied action or AoO, for instance), then give up your 3rd attack to give yourself 16 'protection' until the start of your next turn. Or, you could set it so that iterative penalties on the attack given up reduce the 'protection' too, so in the above example, it would only provide 6 'protection' unless you gave up your 1st attack to get 16.