How about something for a DnD Next Fighter?
When you hit an enemy, you may tare and destroy the armor instead of doing full damage. When you hit an enemy, you may not add your ability modifier to the damage to impose a -1 penalty to the armor class of the enemy. This penalty stacks with itself, but only be as large as your proficiency bonus. As an example, if your proficiency bonus is +2, you may only impose a -2 penalty to armor this way.