Why would you be sorry for starting with Claws?
Anyway, here's what I had in mind for the first power. I'm currently wondering whether to go with this or not in light of the new suggestions.
Damage Shield [-1]
Description: For whatever reason, attacking you isn't safe. Maybe you're covered in spikes, or maybe your body flows with 10 000 volts of electricity.
Musts: Nothing in particular.
Effect:
Damage Shield. Whenever a character makes an unarmed attack against you and misses, they take physical stress equal to the number of shifts by which your defence roll exceeds their attack roll. This might also trigger on some maneuvers, if the GM deems it appropriate.
Conductive Damage Shield [-1]. This power works against against all melee attacks, not just unarmed ones.
Reflective Damage Shield [-1]. (Requires Conductive Damage Shield) This power works against all attacks, not just melee ones.
Dangerous Damage Shield [-1]. This power inflicts two additional stress whenever it triggers.
Lethal Damage Shield [-1]. (Requires Dangerous Damage Shield) This power inflicts a further two additional stress when it triggers.
Mutual Damage Shield [-1]. (Requires Dangerous Damage Shield) This power triggers when you are hit as well as when you are missed. Treat the attacker's threshold shifts as negative shifts for the purposes of calculating this power's damage.
I think it might be a little underpowered.
Bibliophile has the right idea for what I intended for the second power. I'm having trouble deciding on a difficulty for the defence roll.