You know there has been a game design feature been done in RPGs for a very very very long time. Which I never understood why FPS never adopted, especially the more realistic ones with normal soldiers with no special powers of any sort.
Is Critical Wounds.
Get rid of the regenerating health going on in FPS and add some stragety. Bring back medpacks and add in a feature of critical wounds. One of the reasons why FPS don't always generally appeal to me is one how easy it is to modown people with a gun and how easily they die. But there has always been something that bugs me a great deal too. When your main character falls from explosions and high buildings, etc.
So let's say you and your squad were tricked, the roof explodes and you all fall down.
A Critical Wound would mean the next mission till the end of this chapter, you would be playing the game with half health, health packs would stop the bleeding from the wound, and would have a left for dead painkillers for other reasons.
I think it add more stragety and more challenge, but maybe that's just me.