The health system as it stands now is a little to all or nothing, either you can use your body parts just fine without any penalties regardless of how injured they are or they are immediately broken and completely unusable.
A system to simulate more gradual and long-term healing as well as permanent damage to a body part could add more realism as well as open up more opportunities to make curtain effects/mechanics more dangerous and consequential.
We could do this by adding a second HP pool that works as a cap for the current HP pool with both pools being equally large. The current HP pool would represent more superficial damage like open wounds, cuts and bruises that can be healed fairly quickly and with the help of healing items. The second pool would represent more long-term/serious damage and accumulative damage/part of the healing process that just take longer time to complete.
The damage to the second HP pool would be 1/10 of the damage taken by the normal HP pool until the second HP pool reaches 0 and any damage taken after that will go directly to the second HP pool to represent you really getting beaten up and injured as a result. Damage to this second HP pool should also come with debuffs to the effected body part, possibly similar to the ones for encumbrance since both injuries and encumbrance would hinder the functionality of a body part. The second HP pool would heal at the same rate as the normal HP pool but would not be affected by healing items.
There would also be a third HP pool for the limb which represents damage beyond the healing capability of the human or permanent damage but this pool only starts taking damage after the second HP pool has reached 0. This pool would only be there for the limb since you can take horrendous damage to the limbs and still survive, if the second HP pool reaches 0 for the torso or head you DIED. This pool would not naturally regenerate and would act as a cab for the second pool. If this pool reaches 0 than this limb becomes mangled and permanently unusable.
There should be a few ways to heal permanent damage but these should be very hard to get. One should be surgery since this can restore some functionality and reverse some damage in a limb up to a point maybe up to 20% of the third health bar can be restored once to represent the damage that can be fixed with surgery, this could be done by a autodoc or surgeon with the former being cheaper and easier but more unreliable to find and the latter being expensive and harder to find but more reliable. Another would be certain mutations like (lizard) regeneration that can heal permanent damage slowly over time maybe at 1/10th the normal rate. Another would be stem cell therapy (that stem cell machine or a new stem cell shot item that is extremely rare and heals a set percentage).
This system could also be used to make new mechanics or old mechanics more interesting or dangerous and realistic by making them deal damage to the second HP pool instead to the normal HP pool. (make frostbite and fire deal direct damage to the second HP pool since you can’t easily heal those wound with healing items and making them more of a real threat). And we could keep broken limbs mechanics as It is now but only break limbs when you take a ridiculous amount of bash damage in one hit this would make car crashes and jumping of rooftops as well as fighting certain enemies (hulk smashes you against wall and breaks both your legs) in melee more realistic and dangerous since you now you could break your arm even if you didn’t lose all your HP. It would also be far less work than the DF like system that has been proposed before sometimes and build on the current system in place.