By this do you mean the ability to heal when you kill an enemy?
I actually think that that would be a really good idea...
Like you when you kill a soldier you steal their armor plates...
That would be great! I'm a more gameplay preferable person. So I mean I will like it more if you make the shooting parts more thrilling like dodging, longer shooting range, cooler blood/bullet effects, melee, and add enemy delayed attack so you can move out of their distance or walk through them without being attacked instantly. Anything that make you have the feeling to kill more . . .