Its been a long way since I played X-COM for the last time, but from what I remember, the damage system with medikit usage included was very nice. I'm aware this game is not a copy of old X-COM, but it can give us some ideas. I'll list how I remmember it (% values are just guesses. As for the rest of it, please correct me if I'm wrong):
-ANY damage would lower all soldier stats for the rest of the battle, in a random (but still proportional to the amount of damage taken) magnitude for each stats. Stat loss was cumulative for each hit taken. Medikits wouldn't help with that stat loss (remember this loss is just until the end of the mission or until the soldier gets fully healed at the hospital).
-ANY damage would have a chance of inflicting fatal wounds of some level, but engine worked in a way that it was not that common to happen and when it did, in most cases was related to a massive loss of HP. Depending on the magnitude of the damage, it would toke up to 3 levels of fatal wounds on a single, massive damage dealing shot.
-Fatal wound system was simple: On the end of each player round (when clicking the end turn button), the soldier on fatal wound takes 1*(lvl of fatal wound) damage until he dies, mission ends or is healled by a medikit.
-Medikit interface did have a screen that displayed the hurt body parts, but it was just for the realistic feel and didn't really matter for any effect of the stat loss or healing system. It just was cool to see a random limb, head or torso in need of treatment, improving sense of reality.
-Medikit had 3 modes: one for improving lost morale one to wake stunned soldiers, and one for healing fatal wounds.
-Fatal wound heal required one use of the medikit per level of fatal wound and restored a small amount of HP (usually no more than 20% of all damage taken once all levels of wounds was patched up). It had no effect if no fatal wound was inflicted or if it was already healed (thus preventing medikit abuse).
-I don't remember any limit on morale raising mode usage.
-"Unstunning" mode simply healed a random small amount of stun damage (and ONLY stun damage) each use. When stun damage<current HP, soldier wakes up
-From what I remmember, every hit would deal, as well as its regular damage on its damage type category, a small amount of stun damage (10%-30%). If stun damage at any moment heppened to be = or > than CURRENT HP (not max) of the soldier, it would faint. The same for aliens. Some weapons like explosive types would be better to stun a target with the same amount of real damage dealt, rather than other weapons. This way there was always a small chance of getting a live alien even without any "stun equipment".
This info is probably not very accurate, but this should help discuss the damage/stat loss/fatal wound system and the medikit usage.
IMHO, this system is very good and balanced, being realistic, simple and preventing medikit abuses at the same time.