A few ideas:
Smack down: Attack with shield to knock target prone.
Bonuses to bull rushes with shields.
Lessened ACP for shields
Cover other: Defensive action providing cover based on the shield type to you and those in the square directly behind you.
Dual wield shields: you can wear a buckler on your main hand along with a light weapon and any other type of shield with your off hand. Gain both AC's. DWP apply.
Stuck weapons: slashing and piercing weapons can get stuck in wooden shields with a special sunder attack in response to an attack.
Shield surfing: really specific I know but it be cool when used. Probably not in actual water though.
Shield Drop: Jump onto foes with your shield(s) at his feet. Could knock prone and deal falling damage and prevent your own. Of course a balance check would be needed.
Skull bash: Take shield in both hands, (or still equipped if a buckler) to a the head. reduced armor bonus (its only the helmet), penatlies (for a specific target). can produce effects like staggered or stunned.
Razored shield: not a class ability but a new kind of shields with sharp edges. I think its been made but I'm not sure. Allows slashing damage or piercing for the corners. And of course a really deadly frizbee with bucklers. Give it vorpal and you have a shield that can decapitate your foes.
Shield knotches: again not a class feature but soemthing to give to sheilds. It can allow you to catch a slashing weapons blade in notches in the shield allowing a disarm attempt. You'd get bonuses like a locked gauntlet because most shields are afixed like that.
Quicker shield equiping/dequiping
Thats about all I can think of.