Super simple called shot idea:
Called Shot (Hinder): When you make an attack, if you do not have disadvantage, you may attempt to make a called shot (hinder) on your target. You suffer disadvantage on your attack, but if you hit your target, your target is suffers disadvantage on the next attack roll or ability check it makes before the start of your next turn.
Called Shot (Critical): When you make an attack, if you do not have disadvantage, you may attempt to make a called shot (critical) on your target. You suffer disadvantage on your attack, but if it hits the target, you score a critical hit.
These are both great, and well written. The "Critical" option accomplishes the goal. It also sets a good pattern for other combat maneuvers, such as disarm.