Let me start of by saying I love the idea of the passive shield attack.
The issue I see is lack of damage and DR bypass of shields. I think passive shield bash should work like glancing blows.
Shield bash (free): You have a 5% chance to add a shield bash into the attack chain. The shield bash damage is Shield damage + 10% of weapon damage.
Improved Shield bash: You gain a 15% chance to attack with your shield (20% total). The damage improves to Shield base + 25% weapon damage.
This would mean when the shield triggers if your main hand can bypass good and silver DR, then so can the shield. Glancing Blow damage would still be better due to the magic chance. TWF would be better due to how often it triggers.
Other ideas:
- Active shield bashing gains a bonus chance to stun enemies and ties in with stunning blow feat. If you have both maybe your shield gains a bonus like a stunning weapon. Has a x% chance to stun on passive attacks.
- Shield KDs.