General idea - Make stim packs less useful. If you step on a land mine in middle of combat, you cant just pop open your inventory and heal your shattered legs.
My thoughts on how to implement it - Only two ways to heal cripple limbs
1. Visit the doctor
2. Use a first aid kit outside of combat
3. Med-x can give temporary relief to everything but head trauma. (If you have ever been on morphine, you know it can get you past an obscene amount of pain)
Why the need for this mod? Stim packs are just to useful, even on survival. How many of us got crippled, and are not phased by it in the least? All it takes is to pop a stim, and your back in the fight. This would also make some of the perks that reduce limb damage far more useful.