Some rusted iron arm, which can move just like a normal arm, no more, no less, but is rusted because of the sand, water, etc... and is more coherent in Kenshi's universe.
We could have some different levels of prosthetic limbs, depending on the research lvl of your faction, allowing more "hp" to said limb (instead of 100 for right arm, you'll get 125 for instance, or 75 if it's a lower lvl limb), or a wood leg with no more than 50 HP, making the character always limp, but at least able to walk, allowing more differents styles of limbs (big ones, clean, rusted, crappy, woody leg, etc...)
This, leading to permanent scars and injuries on flesh and bone limbs, lowering the maximum of HP of the limb, obligating people to replace sometimes a limb with a mechanical one
This, also leading in a new stat, like mechanics, who could repair damaged mechanical limbs, and medic (no field medic, but real surgeon) who could heal a not too much injuried limb, restoring it's max HP closer to 100
Just throwing ideas. I got tons like that
