Called shots (5e quick fix)

5e quick fixes are exactly what they say on the tin. Small house rules to fix D&D problems you probably don't have. Use them wisely!

D&D has no need of "called shots", but I like them anyway. 5e has no explicit universal mechanics for attacking an arm or leg instead of the torso or head; it is assumed that you're doing your best to wear down your foes with each attack. Some features, feats, spells etc., might have more specific goals, but this will only help certain characters.

If you want concrete and universal mechanics for EVERY character, the solution is quite easy.

Assume everyone can do everything - some PCs just do it better. This is heavily implied in 5e IMO. For example, anyone can trip, shove or disarm; the Battle Master just does it BETTER (same goes for the Sharpshooter and many other feats, as you'll see).

If you get the list of maneuvers from the Battle Master, you have a good guide of things that might be possible to anyone. Just tone them down to keep the Battle Master special. Some things to keep in mind: nothing should be free, and most maneuvers will be sub-optimal unless you're using them in the right circumstances. The idea is not chopping everyone's legs so you can win quickly, but chopping the Wendigo's leg so it cannot run away!

Which means: get rewarded for cleverness and awesomeness, not for system mastery and number-crunching.

If nothing is free, what is the cost? Maybe a bonus action, a reaction, half damage, disadvantage*, or  a combination of those.

* The caveat with disadvantage is that it can be abused; if you're not using multiple advantages, try a -5 penalty instead.

Quick example: attack the arm. Let us say your goal is disarming your enemy and ALSO causing damage. This maneuver should be worse than a Battle Master's Disarming Attack but ALSO no better than the optional disarming rule in the DMG (or else, why you you use that rule?).

One option: attack, and if you hit you deal half damage, AND the target must make a Strength (Athletics) check or Dexterity (Acrobatics) check against your roll* or be disarmed.

*(You might use your "save DC" instead: 8 + proficiency bonus + ability modifier. This ensures the target has a good chance of resisting even if the attacker rolls well, which balances things. Of course, opposed rolls make things easier...  and if you roll a natural 20 it is cool to see your manuever succeed!)

Same thing goes for attacking the legs, or feet. Do some damage and trip your foe.

What about head shots? Well, it depends.

If you just want more damage, use the Sharpshooter feat as a guideline: someone without the feat can take -5 to the attack and +5 to damage. Do not extrapolate from there, because it quickly becomes a math exercise. This should only be used in very specific circumstances; otherwise, just assume you're doing your best to deal the most damage. As a general rule, I would advise against allowing it.

Stunning or knocking out you enemy is better. Let's take the monk's Stunning Strike as an example. No "half damage" here, it wouldn't make much sense when attacking the head. Just roll with disadvantage, and if you hit the target makes a Constitution saving throw against your save DC to avoid getting stunned until the beginning of your next round (not the end, or it would be almost as powerful as a 5th level monk spending ki!). Knockout is an alternative to decapitation (see below).

Optional caveat: the target suffers NO effects if the original damage caused (before halving, etc.) is less than 20% of its HP. This is meant to avoid disarming a Fire Giant with 2 points of damage (you would need 17 damage, which is not that easy for most characters).

Optional addition: if the original damage caused is equal to HP, you get the D! Which means dismember, disembowel or decapitate, with no save (not even death saves!). It only works if all the damage comes in a single turn. This is cool to use against minions, but not useful against big bosses. So you can diminish the damage needed to chop off smaller appendices: maybe 10% HP is all that is needed to cut one of the beholder's eyestalks. A hand might be twice as frail as an arm. The hydra has its own rules, of course.

Anyway, here is the complete system in one single table. You can extrapolate other maneuvers from there.

	Penalty	Effect	To avoid effect...	The D!
Arm	Half damage.	Disarmed	Acrobatics/Athletics	Dismemberment
Leg	Half damage.	Prone	Acrobatics/Athletics	Dismemberment
Head	-5 to hit	Stunned (1 round).	Constitution save.	Decapitation
Vitals	-5 to hit	+5 damage	None.	Disembowelment

EDIT: on a second thought, it would be nice to have a middle tier of damage (broken arm, exhaustion, etc.) Anda maybe a 1/4 HP, 1/2HP and 3/4 HP progression. And a formula dividing HP by the number os appendices. And... Oh well, back to the drawing board!