There seems to be pretty widespread consensus on the boards that shields suck, despite efforts by the PF designers to create a series of feats for shields. I agree that shields in PF (ever since 3.0, really) can't compete with dual-wielding or even two-handed weapons. This is sad for people like me who love the traditional image of the knight with sword-and-shield. So I've worked on a feat meant to give shields a special, useful defensive edge...but hopefully not unbalancing or confounding existing mechanics. Any constructive criticism would be helpful and appreciated. Thanks!
Shield Block (Combat)
You use your shield to knock aside an attack that otherwise might hit you.
Prerequisites: Proficient with shields, Shield Focus, base attack bonus +1.
Benefit: When you are wielding a shield with which you are proficient and would be hit by a physical melee or ranged attack targeting you, you may roll your CMB plus a bonus based on the kind of shield you are wielding (+0 for a buckler, +1 for a light shield, +2 for a heavy shield, +4 for a tower shield) against the attacker’s CMD. If you succeed, immediately apply a +2 (if wielding a buckler or light shield) or +4 (if wielding a heavy or tower shield) cover bonus to your AC against this attack. This benefit does not stack with a cover bonus provided by any other source, nor does it apply against area-effect attacks or damage. Attempting to use shield block (whether or not it succeeds) uses an attack of opportunity for the round.
Normal: Shields do not provide cover.
Yes, your summary is correct. No, the enemy would not reroll the attack (as I thought this would just add another level of complication and variability on top of the original attack roll and combat maneuver check). If the CMB check succeeds, the +2 or +4 would be added retroactively to protect against that one attack that otherwise would have hit.
Thanks, Thomas. I thought about these ideas, but I wasn't sure for a couple of reasons. First, is there any other mechanic in PF in which CMB is rolled against something besides CMD? I wanted to ground the shield block in a core mechanic rather than an ad-hoc formulation. Second, tying the block's odds of success to the attack roll will tend to be self-defeatingly circular, since if the attack roll is high enough the cover bonus won't do enough to block the attack anyway.
Your other idea of just using the shield's AC bonus is also something I considered but wasn't sure about for a couple reasons. First, there would be no difference between a buckler and a light shield (but maybe this shouldn't be a real concern?). Second, if the shield had an enhancement bonus would it be added too? My fear is that adding a shield's enhancement bonus (especially once you get a +3 shield or better) would make the block roll exceptionally easy to succeed.
If a case can be made against my reasoning, though, I'm very open to hearing it. Maybe there's an angle I'm missing? Much appreciated!
And here is the improved level of the feat that I've been playing with...but I am even less certain if it is balanced or useful.
Shield Block, Greater (Combat)
You use your shield to knock aside an attack that otherwise might hit an ally.
Prerequisites: Proficient with shields, Shield Block, Shield Focus, base attack bonus +6.
Benefit: As an immediate action, you may use the benefit of your Shield Block feat against a physical melee or ranged attack targeting a willing person adjacent to you. Doing this also counts as an attack of opportunity by you.
Normal: Shields do not provide cover.
Maybe something similar to the crane wing feats, like once per round as an immediate action you can add the cover bonus the shield would apply against an attack? Would streamline things a bit, but dunno if that might be something you are looking for.