I made a shield guy a long time ago. I'll try to remember the stunts he had and throw in any additional ones I can think of:
Defender: If an ally in the same zone is hit with an attack, you can use your shield to block the attack. It uses your next turn to do so.
(note: based on the Riposte stunt. Originally, I had it so you had to roll a defense against the attack, but now that I look at it, since it uses your next action - IE: only once/round - for something that is, essentially, a reaction, I think it should just be auto-success. Thoughts?)
Thrown Defense: (must have Defender) Same as Defender except you can defend an ally within up to one(or two?) zone(s) away. You must throw your shield for this to work.
Mighty Leap: (must have Defender and a speed power) Same as defender but you may also move your free supplemental move granted from your speed power to defend an ally. (Example: supernatural speed lets you move 2 zones for free. provided there are no borders, you can defend an attack against an ally up to 2 zones away. It uses your next action to do so.)
Shield Master: You are an expert at using a shield. Add +2 to your defense and blocks when using a shield.
Save the Children! When doing a block with your shield, you may protect all allies within a zone. (by the rules, you can already affect multiple people with a single block, so maybe it's a useless stunt. It might need an extra boost.)
Boomerang Shield: when you use your shield as a thrown weapon, it comes back to your hand.
Deflect Bullets: If you successfully defend against a ranged attack with your shield, you may use your next action to immediately make an attack against another enemy within the same number of zones that the attack came from. Use your weapon skill to roll accuracy and use the weapon value of the original attack.