Was going to post a very similar idea, but I did a search beforehand and found that you beat me to it by a month or so...
Nonetheless... my gaming PC died recently, and sitting here waiting for the new one to arrive, I've been "thinkin" (aka coming up with ddo stuff 'cause i can't play right now).
and I've been thinking about AC and shields... here's what I came up with for shields which is similar to your idea OP, with a couple of adjustments to ensure the power scales with level:
NEW EFFECT - SHIELD DEFLECTION
*Available to any character with Shield Proficiency (or Tower Shield Proficiency)
*Characters can choose to actively use their shield to Deflect incoming attacks with a toggle that is usuable with any other stance.
When active, the shield will Proc a "Shield Deflection" at a frequency equal to 10% plus the defender's BAB.
e.g. A lvl 1 fighter using a shield will Proc a shield deflection on 11% of incoming attacks.
e.g. A lvl 20 fighter using a shield will Proc a shield deflection on 30% of incoming attacks.
The base chance is identical for all shield types.
Effect #1
If the Shield Deflection procs, the attack will be partially or entirely deflected by the shield as follows:
Take the shield's normal AC bonus + the defender's BAB and add the result to the defender's Armour Class for the purposes of determining whether this attack is a hit.
and...
Effect #2
If the blow Hits anyway:
apply untyped stacking DR equal to the shield's blocking DR
Notes:- Shield Deflection can be procced by missile spells, but not by direct attack spells or aoe spells. For missle spells, as there is no to-hit roll, the shield provides its Deflection DR only.
- The Deflection Roll will proc as normal, irrespective of whether character is blocking. And the extra DR stacks with blocking DR.
- This is NOT overpowered compared with Blur or Displacement which are 20% and 50% full miss chance
- Using BAB allows the ability to scale with fighting ability.
On the BAB scaling:
Using BAB means that a lowbie sword & board fighter has a relatively small chance to proc a deflection roll, and it will provide a relatively small AC boost versus that attack:
e.g. 3rd lvl fighter with a +2 Tower Shield will have a 13% chance to proc a Deflection, which would provide:
+9 AC vs that attack AND +4 DR if it hits
Meanwhile at high level the effect procs more often and is more potent.
e.g. 20th lvl fighter with a +5 Tower Shield will have a 30% chance to proc a Deflection which would provide:
+29 AC vs that attack AND +4 DR if it hits
And finally, some extra fun bits for magic shields:
New Magical Shield Suffix - "Deflection" 1 to 5
While used for Shield Deflection, this shield provides 1% - 5% bonus to the chance to proc a Shield Deflection and adds 1-5 to the stacking DR on deflected attacks.