Hit Locations in Fate
So, hit locatons are a thing that lots of people deride in RPGs, and often they become pretty convoluted, extra rolls, weird HPs, and generally a bunch of different little fiddly bits. Things like the WOne Roll Engine and Burning Wheel manage this pretty well in my opinion (basically - the damage roll is to a specific location, or the damage is to a specific location set by the target, with a small things to move it around).
Next, and importantly, one of the hardest parts of Fate combat is Consequences. Basically, they are small aspects added to the scene, and often go overboard. One of the complicated thing is when someone decides to take the Minor Consequence of "Broken Ribs" and then suddenly realizes that those ribs are healed in the next scene or so, and complains that it's unrealistic for them to heal so fast. Or, worse, someone spends three or so minutes trying to figure out a good consequence and kills the action's tempo. Both of those kinda hurt the mood.
So my goal is aspects that quickly communicate how bad they are, as well as quickly be defined. During a combat you want to make sure that the specific actions are easy to define.
So the simplest form of this is basically: Leg is adjacent to Leg and Torso. Torso is adjacent to Head, Arm, Arm, Leg Leg, Arm is adjacent to Torso, and Head is adjacent to Torso (obviously people have two legs and two arms). So that's a pretty quick picture. Whenever a character takes damage, they get hit in the location they specify. So, for exmaple, Jenn attacks Liam with a sword, Liam calls that it's going into his left arm. Then, in this example, Jenn could invoke a specific aspect related to targeting or position to move that one step. If required she could spend a second invoke to move it again. This is how a sniper would move a shot from the leg to the head, using two "Aiming" invokes.
So the next thing is "Why would I use a hit location?!". The first step is to place a Minor, Moderate, Severe, and Extreme consequence in each hit location. Call them (for the sake of arguement) Hurt, Damage, Pulped, Destroyed. Hurt clears up pretty quickly, Destroyed gives you a permanent aspect related to that thing missing. This means that a character will want a specific location to be hit a little each, whereas the attacker will want each locaton to be hit as many times as possible (obviously destroying the Torso or the Head will kill the person). You can make this deadlier by removing some conditions from some locations (I recommend the Minor from the Head at the very least).
But that's not the next place to go, there's a few places that I start thinking. The first is Floating Conditions, which are like more common conditions. In a ship conflict during a space opera battle you might have the condition Energy Shields, and it's a floating Condition, which applies once, but to every hit location (so the first time you are hit you can ignore 2 stress using that condition). So - this allows you to have durable but ablative damage in Fate without using quite the same mechanics as Stress. Typically I would minimize the amount of stress someone has, but instead give them more conditions (if I was going to use this variant). So a monster might have features like Covered in Ice where it has an applative armour that can be chipped away. That's a pretty basic monster.
But, if Conditions are aspects that turn on when a threshold is reached, what if I have a stunt "Because I am the Hulk, I get +2 to Forceful when I have the condition Enraged". So when I get a mental condition, a stunt engages. What about if a Wizard has the stunt "When I am Hurt in my torso or head, I get +1 to physical defense rolls". And generally you can use this to model powers off of. But we can go further than that!
What about aspects that turn off when the condition is met. So you have an aspect "Warrior's Honour" that applies, until you use it to take stress. Meaning that you can lose community respect, in exchange for avoiding some damage (also, can't be compelled to not kick some sand in the opponent's face). From there you can make more obvious things available like the Aspect "Tentacle" with the skill "Whip and Snare +2" which a monster has four of. They are both conditions, and also hit locaitons, and consequences. By attacking the monster, you can hit it in a tenatacle, which will absorb some stress then be destroyed, or the Bulbous Torso which has many conditions inside it, but will kill it outright (of course, unless you take time to aim, you aren't able to pick where you attack).
Ok - so the main things we now have:
Off Aspects - aspects that stop existing once a certain amount of stress has been put into them
On Aspects - traditional Fate Toolkit conditions
Aspect Stunts - stunts that only exist when you have or don't have a specific set of aspects
Hit locations - typically the limbs, torso, and head, but could go a little crazy for non-humanoid creatures.
Hit Location Skills - used to make a large monster threatening, and to sell its size, or to otherwise make a creature more capable while making it also reducible.
Floating Conditions - conditions that you can automatically use before you get into the fiddly bits of combat.
Give the character 0 stress +1 for every odd Physique, a minor, moderate, severe, and extreme in all the hit locations (except only moderate and extreme in the head), as well as charge them a Stunt to add a condition to their character, and away you go!