I have been thinking about cybernetics for a while now, ever since i played (the dissapointing)space seige . Although space seige had some great ideas, sadly they were poorly implemented. 

first
Limb Damage 

How mucy  damage can a limb or part take before its completly useless and needs replacing (arms ,legs,hands ,torso,ect). damaged caused buy dying or critical incoming damage.

Limb Replacement

Once the limb or part starts becoming below par with the normal standard its time to replace that limb. Then you travel to the nanite machine where you lie down and the machine encases your body. Then you order the  replacemet limb. Depending on how much you invest in the limb or how much optional crafting material you add, you could achieve many varied results.
 Maybe it could fit under the current inventory system as well.
Just a quick idea.